This article was reported by Judith Miller, Jeff Gerth and Don Van Natta Jr. and written by Ms. Miller

Inside the White House situation room on the morning terrorism transformed America, Frank Miller, the director for military affairs, was suddenly gripped by a staggering fear: "The White House could be hit. We could be going down."

The reports and rumors came in a torrent: A car bomb had exploded at the State Department. The Mall was in flames. The Pentagon had been destroyed. Planes were bearing down on the capital.

The White House was evacuated, leaving the national security team alone, trying to control a nation suddenly under siege and wondering if they were next. Mr. Miller had an aide send out the names of those present by e-mail "so that when and if we died, someone would know who was in there."

Somewhere in the havoc of the moment, Richard A. Clarke, then the White House counterterrorism chief, recalled the long drumbeat of warnings about terrorists striking on American soil, many of them delivered and debated in that very room. After a third hijacked jet had sliced into the Pentagon, others heard Mr. Clarke say it first: "This is Al Qaeda."

An extensive review of the nation's antiterrorism efforts shows that for years before Sept. 11, terror experts throughout the government understood the apocalyptic designs of Osama bin Laden. But they never reacted as if they believed the country was as vulnerable as it proved to be that morning.

The defensive work of tightening the borders and airport security was studied but never quite completed. And though the White House undertook a covert campaign to kill Mr. bin Laden, the government never mustered the critical mass of political will and on-the-ground intelligence for the kind of offensive against Al Qaeda it unleashed this fall.

The rising threat of the Islamic jihad movement was first detected by United States investigators after the 1993 bombing of the World Trade Center. The investigation of that attack revealed a weakness in the immigration system used by one of the terrorists, but that hole was never plugged, and it was exploited again by one of the Sept. 11 hijackers.

In 1996, a State Department dossier spelled out Mr. bin Laden's operation and his anti-American intentions. And President Bill Clinton's own pollster told him the public would rally behind a war on terrorism. But none was declared.

By 1997, the threat of an Islamic attack on America was so well recognized that an F.B.I. agent warned of it in a public speech. But that same year, a strategy for tightening airline security, proposed by a vice- presidential panel, was largely ignored.

In 2000, after an Algerian was caught coming into the country with explosives, a secret White House review recommended a crackdown on "potential sleeper cells in the United States." That review warned that "the threat of attack remains high" and laid out a plan for fighting terrorism. But most of that plan remained undone.

Last spring, when new threats surfaced, the Bush administration devised a new strategy, which officials said included a striking departure from previous policy — an extensive C.I.A. program to arm the Northern Alliance and other anti-Taliban forces in Afghanistan. That new proposal had wound its way to the desk of the national security advisor, Condoleezza Rice, and was ready to be delivered to the president for final approval on Monday, Sept. 10.

The government's fight against terrorism always seemed to fall short.

The Sept. 11 attack "was a systematic failure of the way this country protects itself," said James Woolsey, a former director of central intelligence. "It's aviation security delegated to the airlines, who did a lousy job. It's a fighter aircraft deployment failure. It's a foreign intelligence collection failure. It's a domestic detection failure. It's a visa and immigration policy failure."

The Clinton administration intensified efforts against Al Qaeda after two United States embassies in Africa were bombed in 1998. But by then, the terror network had gone global — "Al Qaeda became Starbucks," said Charles Duelfer, a former State Department official — with cells across Europe, Africa and beyond.

Even so, according to the interviews and documents, the government response to terrorism remained measured, even halting, reflecting the competing interests and judgments involved in fighting an ill-defined foe.

The main weapon in President Clinton's campaign to kill Mr. bin Laden and his lieutenants was cruise missiles, which are fired from thousands of miles away. While that made it difficult to hit Mr. bin Laden as he moved around Afghanistan, the president was reluctant to put American lives at risk.

But a basic problem throughout the fight against terrorism has been the lack of inside information.

The C.I.A. was surprised repeatedly by Mr. bin Laden, not because it failed to pay attention as much as because it lacked sources inside Al Qaeda. There were no precise warnings of impending attacks, and the C.I.A. could not provide an exact location for Mr. bin Laden, which was essential to the government's objective of killing him.

At the F.B.I., it was not until last year that all field offices were ordered to get engaged in the war on terrorism and develop sources. Inside the bureau, the seminars and other activities that accompanied these orders were nicknamed "Terrorism for Dummies," a stark acknowledgment of how far the agency had not come in the seven years since the first trade center attack.

"I get upset when I hear complaints from Congress that the F.B.I. is not sharing its intelligence," said a former senior law enforcement official in the Clinton and Bush administrations. "The problem is that there isn't any to share. There is very little. And the stuff we can share is not worth sharing."

Officials at the Federal Bureau of Investigation and the Central Intelligence Agency said that they had some success in foiling Al Qaeda plots, but that the structure of the group made it difficult to penetrate. "It is understandable, but unrealistic, especially given our authorities and resources, to expect us to be perfect," said Bill Harlow, a C.I.A. spokesman.

The reasons the government was not more single-minded in attacking Al Qaeda will be examined exhaustively and from every angle by Congress and others in the years ahead.

In an era of opulence and invincibility, the threat of terrorism never took root as a dominant political issue. Mr. bin Laden's boldest attack on America property before Sept. 11 — the embassy bombings — came in the same summer that the Monica Lewinsky scandal was engulfing President Clinton. A full fight against terrorism might have meant the sacrifice of money, individual liberties and, perhaps, lives — and even then without any guarantee of success.

Mr. Clarke, until recently the White House director of counterterrorism, warned of the threat for years and reached this conclusion: "Democracies don't prepare well for things that have never happened before."

The First Warning:
A Horrible Surprise At the Trade Center
On Feb. 26, 1993 — a month after Bill Clinton took office, having vowed to focus on strengthening the domestic economy "like a laser" — the World Trade Center was bombed by Islamic extremists operating from Brooklyn and New Jersey. Six people were killed, hundreds injured.

Today, American experts see that attack as the first of many missed warnings. "In retrospect, the wake-up call should have been the 1993 World Trade Center bombing," said Michael Sheehan, counterterrorism coordinator at the State Department in the last years of the Clinton presidency.

The implications of the F.B.I.'s investigation of the bombing were disturbingly clear: A new and dangerous phenomenon had taken root. Young Muslims who had fought with the Afghan rebels against the Soviet Union in the 1980's had now taken their jihad to American shores.

The F.B.I. was "caught almost totally unaware that these guys were in here," recalled Robert M. Blitzer, a former senior counterterrorism official in the bureau's headquarters. "It was alarming to us that these guys had been coming and going since 1985 and we didn't know."

One of the names that surfaced in the bombing case was that of a Saudi exile named Osama bin Laden, F.B.I. officials say. Mr. bin Laden, they learned, was financing the Office of Services, a Pakistan-based group involved in organizing the new jihad. And it turned out that the mastermind of the Trade Center attack, Ramzi Yousef, had stayed for several months in a Pakistani guest house supported by Mr. bin Laden.

But if the first World Trade Center bombing raised the consciousness of some at the F.B.I., it had little lasting resonance for the White House. Mr. Clinton, who would prove gifted at leading the nation through sorrowful occasions, never visited the site. Congress tightened immigration laws, but the concern about porous borders quickly dissipated and the new rules were never put in effect.

Leon E. Panetta, the former congressman who was budget director and later chief of staff during Mr. Clinton's first term, said senior aides viewed terrorism as just one of many pressing global problems.

"Clinton was aware of the threat and sometimes he would mention it," Mr. Panetta said. But the "big issues" in the president's first term, he said, were "Russia, Eastern bloc, Middle East peace, human rights, rogue nations and then terrorism."

When it came to terrorism, Clinton administration officials continued the policy of their predecessors, who had viewed it primarily as a crime to be solved and prosecuted by law enforcement agencies. That approach, which called for grand jury indictments, created its own problems.

The trade center investigation produced promising leads that pointed overseas. But Mr. Woolsey, President Clinton's first director of central intelligence, said in an interview that this material was not shared with the C.I.A. because of the federal rules governing grand jury secrecy.

The C.I.A. faced its own obstacles, former agency officials say. In the wake of the Soviet Union's withdrawal from Afghanistan in 1989, the agency virtually abandoned the region, leaving it with few sources of information about the rising radical threat.

Looking back, George Stephanopoulos, the president's adviser for policy and strategy in his first term, said he believed the 1993 attack did not gain more attention because, in the end, it "wasn't a successful bombing."

He added: "It wasn't the kind of thing where you walked into a staff meeting and people asked, what are we doing today in the war against terrorism?"

Two years later, however, terrorism moved to the forefront of the national agenda when a truck bomb tore into the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995, killing 168 people.

President Clinton visited Oklahoma City for a memorial service, signaling the political import of the event. "We're going to have to be very, very tough in dealing with this," he declared in an interview.

Mr. Panetta said that plans to reorganize the government's counterterrorism efforts were quickly revived. Senior officials recognized that the United States remained vulnerable to terrorism. The bombing proved to be the work of two Americans, both former soldiers, but if Oklahoma City could be hit, an attack by terrorists of any stripe could "happen at the White House," Mr. Panetta said.

Two months after the bombing, Mr. Clinton ordered the government to intensify the fight against terrorism. The order did not give agencies involved in the fight more money, nor did it end the bureaucratic turf battles among them.

But it did put Mr. bin Laden, who had set up operations in Sudan after leaving Afghanistan in 1991, front and center.

Clinton Acts:
A Growing Effort Against bin Laden
As Mr. Clinton prepared his re-election bid in 1996, the administration made several crucial decisions. Recognizing the growing significance of Mr. bin Laden, the C.I.A. created a virtual station, code-named Alex, to track his activities around the world.

In the Middle East, American diplomats pressed the hard-line Islamic regime of Sudan to expel Mr. bin Laden, even if that pushed him back into Afghanistan.

To build support for this effort among Middle Eastern governments, the State Department circulated a dossier that accused Mr. bin Laden of financing radical Islamic causes around the world.

The document implicated him in several attacks on Americans, including the 1992 bombing of a hotel in Aden, Yemen, where American troops had stayed on their way to Somalia. It also said Mr. bin Laden's associates had trained the Somalis who killed 18 American servicemen in Mogadishu in 1993.

Sudanese officials met with their C.I.A. and State Department counterparts and signaled that they might turn Mr. bin Laden over to another country. Saudi Arabia and Egypt were possibilities.

State Department and C.I.A. officials urged both Egypt and Saudi Arabia to accept him, according to former Clinton officials. "But both were afraid of the domestic reaction and refused," one recalled.

Critics of the administration's effort said this was an early missed opportunity to destroy Al Qaeda. Mr. Clinton himself would have to have leaned hard on the Saudi and Egyptian governments.

The White House believed no amount of pressure would change the outcome, and Mr. Clinton risked spending valuable capital on a losing cause. "We were not about to have the president make a call and be told no," one official explained.

Sudan obliquely hinted that it might turn Mr. bin Laden over to the United States, a former official said. But the Justice Department reviewed the case and concluded in the spring of 1996 that it did not have enough evidence to charge him with the attacks on American troops in Yemen and Somalia.

In May 1996, Sudan expelled Mr. bin Laden, confiscating some of his substantial fortune. He moved his organization to Afghanistan, just as an obscure group known as the Taliban was taking control of the country.

Clinton administration officials counted it as a positive step. Mr. bin Laden was on the run, deprived of the tacit state sponsorship he had enjoyed in Sudan.

"He lost his base and momentum," said Samuel R. Berger, Mr. Clinton's national security adviser in his second term.

In July 1996, shortly after Mr. bin Laden left Sudan, Mr. Clinton met at the White House with Dick Morris, his political adviser, to hone themes for his re-election campaign.

The previous month, a suicide bomber had detonated a truck bomb at a military barracks in Saudi Arabia, killing 19 American servicemen. Days later, T.W.A. Flight 800 had exploded off Long Island, leaving 230 people dead in a crash that was immediately viewed as terrorism.

Mr. Morris said he had devised an attack advertisement of the sort that Senator Bob Dole, the Republican candidate, might use against Mr. Clinton and had shown it to a sampling of voters. Seven percent of those who saw the ad said they would switch from Mr. Clinton to Mr. Dole.

"Out of control. Two airline disasters. One linked to terrorism," the advertisement said. "F.A.A. asleep at the switch. Terror in Saudi Arabia."

Mr. Morris said he told Mr. Clinton that he could neutralize such a line of attack by adopting tougher policies on terrorism and airport security. He said his polls had found a reservoir of support for tightening security and confronting terrorists. Voters favored military action against suspected terrorist installations in other countries. They backed a federal takeover of airport screening and even supported deployment of the military inside the United States to fight terrorism.

Mr. Morris, who has since become a columnist and a sharp critic of Mr. Clinton, said he tried and failed to persuade the president to launch a broader war on terrorism.

Mr. Clinton declined repeated requests for an interview, but a spokeswoman, Julia Payne, said: "Terrorism was always a top priority in the Clinton administration. The president chose to get his foreign policy advice from the likes of Sandy Berger and Madeleine Albright and not Dick Morris."

On July 25, a day after Mr. Clinton was shown the effects of the hypothetical Dole attack ad, he announced that he had put Vice President Al Gore at the head of a commission on aviation safety and security.

Within weeks, the panel had drafted more than two dozen recommendations. Its final report, in February 1997, added dozens more.

Among the most important, commission members said, was a proposal that the F.B.I. and C.I.A. share information about suspected terrorists for the databases maintained by each airline. If a suspected terrorist bought a ticket, both the airline and the government would find out.

Progress was slow, particularly after federal investigators determined that the crash of T.W.A. Flight 800 resulted from a mechanical flaw, not terrorism.

The commission's recommendation languished — until Sept. 11. On that day, two people already identified by the government as suspected terrorists boarded separate American Airlines flights from Boston using their own names.

That morning, no alarms went off. The system proposed by the Gore commission was still not in place. The government is now moving to share more information with the airlines about suspected terrorists.

"Unfortunately, it takes a dramatic event to focus the government's and public's attention, especially on an issue as amorphous as terrorism," said Gerry Kauvar, staff director of the commission and now a senior policy analyst at the RAND Corporation.

Targeting Al Qaeda
A Clearer Picture, A Disjointed Fight
As Mr. Clinton began his second term, American intelligence agencies were assembling a clearer picture of the threat posed by Mr. bin Laden and Al Qaeda, which was making substantial headway in Afghanistan.

A few months earlier, the first significant defector from Al Qaeda had walked into an American embassy in Africa and provided a detailed account of the organization's operations and ultimate objectives.

The defector, Jamal Ahmed al-Fadl, told American officials that Mr. bin Laden had taken aim at the United States and other Western governments, broadening his initial goal of overthrowing Saudi Arabia and other "infidel" Middle Eastern governments.

He said that Al Qaeda was attempting to buy a nuclear bomb and other unconventional weapons. Mr. bin Laden, was also trying to form an anti-American front of terrorists that would unite radical groups.

But Mr. Fadl's statements were not widely circulated within the government. A senior official said their significance was not fully understood by Mr. Clinton's top advisers until his public testimony in 2000. Mr. Berger, for example, said he did not recall having been told earlier of Mr. Fadl's revelations.

The war against Al Qaeda remained disjointed. While the State Department listed Mr. bin Laden as a financier of terror in its 1996 survey of terrorism, Al Qaeda was not included on the list of terrorist organizations subject to various sanctions released by the United States in 1997.

The F.B.I.'s counterterrorism experts, who were privy to Mr. Fadl's debriefings, were growing increasingly concerned about Islamic terrorism.

"Almost all of the groups today, if they chose, have the ability to strike us in the United States," John P. O'Neill, the F.B.I.'s chief of international terrorist operations, warned in a June 1997 speech. "They are working toward that infrastructure."

The task, Mr. O'Neill said, was to prevent the next attack, to "nick away" at terrorists' capability of operating within this country. (Mr. O'Neill left the F.B.I. this year for a job as chief of security at the World Trade center, where he died on Sept. 11.)

As Mr. O'Neill spoke in Chicago, the F.B.I. and C.I.A. was homing in on an Al Qaeda cell in Nairobi, Kenya.

The National Security Agency began eavesdropping on telephone lines used by Al Qaeda members in the country. On several occasions, calls to Mr. bin Laden's satellite phone in Afghanistan were overheard. The F.B.I. and C.I.A. searched a house in Kenya, seizing a computer and questioning Wadih El-Hage, an American citizen living in Nairobi who was working as Mr. bin Laden's personal secretary.

American officials counted the operations as a success and believed they had disrupted a potentially dangerous terrorist cell.

They were proven wrong on Aug. 7, 1998, when truck bombs were detonated outside the United States embassies in Nairobi and Dar es Salaam, Tanzania, , killing 224 people, including 12 Americans, and injuring more than 5,000.

Stunned by the plot's ambition and precision, Mr. Clinton vowed to punish the perpetrators, who were quickly identified as Al Qaeda adherents. "No matter how long it takes or where it takes us," the president said, "we will pursue terrorists until the cases are solved and justice is done."

The political calculus, however, had changed markedly since the president's triumph in the fall of 1996, and Mr. Clinton was in no position to mount a sustained war against terrorism.

His administration was weighed down by a scandal over his relationship with a White House intern. Mr. Clinton was about to acknowledge to a grand jury that his public and private denials of the affair had been misleading. Republicans depicted every foreign policy decision as an attempt to distract voters.

Thirteen days after the embassy bombings, President Clinton nonetheless ordered cruise missile strikes on a Qaeda camp in Afghanistan and a pharmaceutical plant in Sudan that officials said was linked to Mr. bin Laden and chemical weapons.

But the volley of cruise missiles proved a setback for American counterterrorism efforts. The C.I.A. had been told Mr. bin Laden and his entourage were meeting at the camp, but the missiles struck just a few hours after he left. And the owner of the pharmaceutical factory came forward to claim that it had nothing to do with chemical weapons, raising questions about whether the Sudan strike had been in error.

The Clinton administration stood by its actions, but several former officials said the criticism had a subtle but serious effect on the pursuit of Al Qaeda. Mr. Clinton became even more cautious about using military force against terrorists, they said.

Unfortunately, the quarry was becoming more dangerous. In the two years since leaving Sudan, Mr. bin Laden had built a formidable base in Afghanistan. He lavished millions of dollars on the impoverished Taliban regime and in exchange was allowed to operate a network of training camps that attracted Islamic militants from all over the world. In early 1998, just as he declared war on Americans everywhere in the world, he cemented an alliance with Egyptian Islamic Jihad, a ruthless and effective group whose leader, Ayman al-Zawahiri, was known for his operational skills.

The Battle Intensifies
Struggling to Focus On `Enemy No. 1'
In the years after the embassy bombings, the Clinton administration significantly stepped up its efforts to destroy Al Qaeda, targeting its finances, plotting military strikes to wipe out its leadership and prosecuting its members for the bombings and other crimes. "From August 1998, bin Laden was Enemy No. 1," Mr. Berger said.

The campaign had the support of President Clinton and his senior aides. But former administration officials acknowledge that it never became the government's top priority.

When it came to Pakistan, for example, American diplomats continued to weigh the war on terrorism against other pressing issues, including the need to enlist Islamabad's help in averting a nuclear exchange with India.

Similarly, a proposal to vastly enhance the Treasury Department's ability to track global flows of terrorist money languished until after Sept. 11. And American officials were reluctant to press the oil-rich Saudis to crack down on charities linked to radical causes.

Still, the fight against Al Qaeda gained new, high-level attention after the embassy attacks, present and former officials say. Between 1998 and 2000, the "Small Group" of the Cabinet-rank principals involved in national security met almost every week on terrorism, and the Counterterrorism Security Group, led by Mr. Clarke, met two or three times a week, officials said.

The United States disrupted some Al Qaeda cells, and persuaded friendly intelligence services to arrange the arrest and transfer of Al Qaeda members without formal extradition or legal proceedings. Dozens were quietly sent to Egypt and other countries to stand trial.

President Clinton also ordered a more aggressive program of covert action, signing an intelligence order that allowed him to use lethal force against Mr. bin Laden. Later, this was expanded to include as many as a dozen of his top lieutenants, officials said.

On at least four occasions, Mr. Clinton sent the C.I.A. a secret "memorandum of notification," authorizing the government to kill or capture Mr. bin Laden and, later, other senior operatives. The C.I.A. then briefed members of Congress about those plans.

The C.I.A. redoubled its efforts to track Mr. bin Laden's movements, stationing submarines in the Indian Ocean to await the president's launch order. To hit Mr. bin Laden, the military said it needed to know where he would be 6 to 10 hours later — enough time to review the decision in Washington and program the cruise missiles.

That search proved frustrating. Officials said the C.I.A. did have some spies within Afghanistan. On at least three occasions between 1998 and 2000, the C.I.A. told the White House it had learned where Mr. bin Laden was and where he might soon be.

Each time, Mr. Clinton approved the strike. Each time, George Tenet, the director of central intelligence, called the president to say that the information was not reliable enough to be used in an attack, a former senior Clinton administration official said.

In late 1998, according to former officials, intelligence agents reported that Abu Hafs, an important Al Qaeda figure, was staying in Room 13 at the Dana Hotel in Khartoum.

With such specific information in hand, White House officials wanted Abu Hafs either killed or, preferably, captured and transferred out of Sudan to a friendly state where he could be interrogated, the former officials said.

The agency initially questioned whether it could accomplish such a mission in a hostile, risky environment like Sudan, putting it in the "too hard to do box," one former official said. An intelligence official disputed this account, saying the C.I.A. made "a full-tilt effort in a very dangerous environment."

Eventually, the C.I.A. enlisted another government to help seize Abu Hafs, a former official said, but by then it was too late. The target had disappeared.

Officials said the White House pushed the Joint Chiefs of Staff to develop plans for a commando raid to capture or kill Mr. bin Laden. But the chairman, Gen. Henry H. Shelton, and other senior Pentagon officers told Mr. Clinton's top national security aides that they would need to know Mr. bin Laden's whereabouts 12 to 24 hours in advance.

"While we were getting some fleeting intelligence — say, where he might be on Aug. 20 — we didn't have the kind of information you'd like to have to mount an operation," said one senior military officer.

Pentagon planners also considered a White House request to send a hunter team of commandos, small enough to avoid detection, the officer said. General Shelton, a former special forces commander, discounted this option as naïve.

General Shelton, the officer said, "always wanted to make sure that if you do something, you do it with sufficient force."

White House officials were frustrated that the Pentagon could not produce plans that involved a modest number of troops. Military planners insisted that an attack on Al Qaeda required thousands of troops invading Afghanistan. "When you said this is what it would take, no one was interested," a senior officer said.

A former administration official recently defended the decision not to employ a commando strike. "It would have been an assault without the kind of war we've seen over the last three months to support it," the official said. "And it would have been very unlikely to succeed."

Clinton administration officials also began trying to choke off Al Qaeda's financial network. Shortly after the embassy bombings, the United States began threatening states and financial institutions with sanctions if they failed to cut off assistance to those who did business with Al Qaeda and the Taliban.

In 1999 and early 2000, some $255 million of Taliban-controlled assets was blocked in United States accounts, according to William F. Wechsler, a former White House official.

Mr. Wechsler said the search for Al Qaeda's assets was often stymied by poor cooperation from Middle Eastern and South Asian states.

The United States, too, he added, had problems. "Few intelligence officials who understand the nuances of the global banking system" were fluent in Arabic. While the C.I.A. had done a "reasonably good job" analyzing Al Qaeda, he wrote, it was "poor" at developing sources within Mr. bin Laden's financial network. The F.B.I., he argued, had similar shortcomings.

Senior officials were frustrated by the C.I.A.'s inability to find hard facts about Al Qaeda's financial operations, such as its annual budget, how it moved money or on what it was spent.

Intelligence officials said the C.I.A. had amassed considerable detail about the group's finances, and that information was used in the broad efforts to freeze its accounts after Sept. 11.

At the State Department, officials reacted sharply to the assault on the embassies. Michael Sheehan, the department's former counterterrorism coordinator, said that after the bombings, Secretary of State Madeleine K. Albright met with her embassy security director every morning and became increasingly focused on efforts to protect her employees and installations.

But to Mr. Sheehan, the response was inadequate. He believed that the United States needed to put terrorism at the top of its foreign policy priorities. It could only be contained if Washington devised a "comprehensive political strategy to pressure Pakistan and other neighbors and allies into isolating not only bin Laden and Al Qaeda, but the Taliban and others who provide them sanctuary," he said, and that did not happen. There were competing priorities. "Our reaction was responsive, almost never proactive."

`We Were Flying Blind'
An Arrest, a Review And New Obstacles
The arrest of Ahmed Ressam was the clearest sign that Osama bin Laden was trying to bring the jihad to the United States.

Mr. Ressam was arrested when he tried to enter the United States in Port Angeles, Wash., on Dec. 14, 1999. Inside his rental car, agents found 130 pounds of bomb-making chemicals and detonator components.

His arrest helped reveal what intelligence officials later concluded was an Al Qaeda plot to unleash at least three attacks during the millennium celebrations, aimed at an American ship in Yemen, tourist sites and a hotel in Jordan frequented by Israelis, and unknown targets in the United States.

"That was a wake-up call," a senior law enforcement officer said, "not for law enforcement and intelligence, but for policy makers." Just as the embassy bombings had exposed the threat of Al Qaeda overseas, the millennium plot revealed gaping vulnerabilities at home.

"If you understood Al Qaeda, you knew something was going to happen," said Robert M. Bryant, who was the deputy director of the F.B.I. when he retired in 1999. "You knew they were going to hit us, but you didn't know where. It just made me sick on Sept. 11. I cried when those towers came down."

A White House review of American defenses in March 2000, after Mr. Ressam's arrest, found significant shortcomings in nearly a decade of government efforts to improve defenses against terrorists at home. The F.B.I. and the Immigration and Naturalization Service, it said, should begin "high tempo, ongoing operations to arrest, detain and deport potential sleeper cells in the United States."

The review called for the government to greatly expand its antiterrorism efforts inside the United States, creating an additional dozen joint federal-local task forces like the one that had been set up in New York. The government, the review said, needed to improve its ability to translate the hours of conversations recorded by law enforcement agencies.

The review identified particular weaknesses in the nation's immigration controls, officials said. The government remained unable to track foreigners in the United States on student visas, despite a 1996 law passed after the first World Trade Center bombing that required it to do so.

In June 2000, after the millennium plot was revealed, the National Commission on Terrorism recommended that the immigration service set up a system to keep tabs on foreign students. Academic institutions opposed the recommendation, fearing that a strict reporting requirement might alienate prospective foreign students, according to government officials. As a result, nothing changed.

As the commission was completing its work, the Sept. 11 hijackers began entering the United States. One of the 19 hijackers, Hani Hanjour, had traveled on a student visa, failed to show up for school and remained in the country illegally.

The F.B.I. had some problems of its own. It had no intelligence warning of an attack on Los Angeles International Airport, which investigators eventually learned was Mr. Ressam's intended target. "The millennium plot, that was a total surprise to us," said a former senior law enforcement official.

Beginning in 1997, senior officials at the bureau had begun to rethink their approach to terrorism, viewing it now as a crime to be prevented rather than solved. But it was the millennium plot that revealed how ill equipped the bureau was to radically shift its culture, former officials say.

It lacked informants within terrorist groups. It did not have the computer and analytical capabilities for integrating disparate pieces of information.

"We did not have any actionable intelligence," one senior official said. "We were flying blind."

In March 2000, Dale L. Watson, the F.B.I.'s assistant director for counterterrorism, started a series of seminars with agents who headed the bureau's 56 field offices, stressing the importance of each office in the fight against terrorism.

Each field office was required to establish a joint terrorism task force with local police departments, modeled after the arrangement begun in New York in the mid-1980's. Field office chiefs were also told to hire more Arabic translators and develop better sources of information. The officials complained that Arabic translations, which were expected to take several days, often took weeks.

Mr. Watson said that the meetings were a centerpiece of efforts to shore up the bureau's counterterrorism capabilities that had begun several years earlier. The meetings, he said, were "designed to bring every office, no matter how small, to the same top terrorism capacities resident in our larger offices like New York."

The F.B.I. renewed its push on Capitol Hill for money to create a computer system that would allow various field offices to share and analyze information collected by agents. Until late last year, Congress had refused to pay for the project.

Without the analytical aid of a computer system, Mr. Bryant said, the bureau's counterterrorism program would be hobbled, particularly if the goal was to avert a crime. "We didn't know what we had," he said. "We didn't know what we knew."

Another former senior official said: "The F.B.I. is the greatest in the world investigating a crime after it happened, but it is not equipped to prevent crimes. It wasn't in the 90's, it wasn't on 9/11."

Overseas, the Clinton administration searched for new ways to obtain the intelligence needed to target Mr. bin Laden. In September 2000, an unarmed, unmanned spy plane called the Predator flew test flights over Afghanistan, providing what several administration officials called incomparably detailed real-time video and photographs of the movements of what appeared to be Mr. bin Laden and his aides.

The White House pressed ahead with a program to arm the Predator with a missile, but the effort was slowed by bureaucratic infighting between the Pentagon and the C.I.A. over who would pay for the craft and who would have ultimate authority over its use. The dispute, officials said, was not resolved until after Sept. 11.

On Oct. 12, an explosive-laden dinghy piloted by two suicide bombers exploded next to the American destroyer Cole in Yemen, killing 17 sailors. Intelligence analysts linked the bombing to Al Qaeda, but at a series of Cabinet-level meetings, Mr. Tenet of the C.I.A. and senior F.B.I. officials said the case was not conclusive.

Mr. Clarke, the White House counterterrorism director, had no doubts about whom to punish. In late October, officials said, he put on the table an idea he had been pushing for some time: bombing Mr. bin Laden's largest training camps in Afghanistan.

With the administration locked in a fevered effort to broker a peace settlement in Israel, an election imminent and the two- term Clinton administration coming to a close, the recommendation went nowhere. Terrorism was not raised as an issue by either Vice President Al Gore or George W. Bush in the 2000 presidential campaign.

In October 2000, the Clinton administration took another shot at killing Mr. bin Laden.

When Mr. Berger called the president to tell him the effort had failed, he recalled Mr. Clinton muttered a curse. "Just keep trying," he said.

The New Team
Seeing the ThreatBut Moving Slowly
As he prepared to leave office last January, Mr. Berger met with his successor, Condoleezza Rice, and gave her a fatalistic warning.

According to both of them, he said that terrorism — and particularly Mr. bin Laden's brand of it — would consume far more of her time than she had ever imagined.

A month later, with the administration still getting organized, Mr. Tenet, whom President Bush had asked to stay on at the C.I.A., warned the Senate Intelligence Committee that Mr. bin Laden and Al Qaeda remained "the most immediate and serious threat" to security.

But until Sept. 11, the people at the top levels of the new Bush administration, if anything, may have been less preoccupied by terrorism than the Clinton aides before them.

At the C.I.A., according to former Clinton administration officials, Mr. Tenet's actions did not match his words. For example, one intelligence official said, the C.I.A. station in Pakistan remained understaffed and underfinanced, though the C.I.A. denied that.

In March, the White House's Counterterrorism Security Group began drafting its own comprehensive strategy for combating Al Qaeda. Mr. Clarke was still nominally in charge, but Bush aides were on the way to approving Mr. Clarke's recommendation that his group be divided into several new offices.

Mr. Bush's principals did not formally meet to discuss terrorism in late spring when intercepts from Afghanistan warned that Al Qaeda was planning to attack an American target in late June or perhaps over the July 4 holiday.

They did not meet even after intelligence analysts overheard conversations from an Al Qaeda cell in Milan suggesting that Mr. bin Laden's agents might be plotting to kill Mr. Bush at the European summit meeting in Genoa, Italy, in late July.

Administration officials say the president was concerned about the growing threat and frustrated by the halfhearted efforts to thwart Al Qaeda. In July, Ms. Rice said, Mr. Bush likened the response to the Al Qaeda threat to "swatting at flies."

The administration's draft plan for fighting Al Qaeda included a $200 million C.I.A. program that, among other things, would arm the Taliban's enemies. Clinton administration officials had refused to provide significant money and arms to the Northern Alliance, which was composed mostly of ethnic minorities. Officials feared that large-scale support for the rebels would also anger Pakistan, which had close ties to its fellow Pashtuns in the Taliban.

President Bush's national security advisers approved the plan on Sept. 4, a senior administration official said, and it was to be presented to the president on Sept. 10. He was traveling that day and did not receive it.

The next day his senior national security aides gathered shortly before 9 a.m. for a staff meeting. At roughly the same moment, a hijacked Boeing 767 was plowing into the north tower of the World Trade Center.