The task, Mr. O'Neill said, was to "nick away" at terrorists' ability to operate in the United States. (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 a 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 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 proved 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 that 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 an effect on the pursuit of Al Qaeda: Mr. Clinton became even more cautious about using force against terrorists.

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 Track 'Enemy No. 1'

In the years after the embassy bombings, the Clinton administration significantly stepped up its efforts to destroy Al Qaeda, tracking 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 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, a Mauritanian and an important figure in Al Qaeda, 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.

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 discounted this option as naA_ve, the officer said.

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.

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 terrorism could be contained only if Washington devised a "comprehensive political strategy to pressure Pakistan and other neighbors and allies into isolating not only Mr. 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," he said.

'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 a Qaeda plot to unleash attacks during the millennium celebrations, aimed at an American ship in Yemen, tourist sites and a hotel in Jordan, 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 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 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. 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, who 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.

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 informers within terrorist groups. It did not have the computer and analytical capacity 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. 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.

Mr. Watson said that the meetings were a centerpiece of efforts to shore up the bureau's counterterrorism work 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."

Overseas, the Clinton administration searched for new ways to obtain the intelligence needed to attack 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 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 cursed. "Just keep trying," he said.

The New Team
Seeing the Threat But Moving Slowly

As he prepared to leave office last January, Mr. Berger met with his successor, Condoleezza Rice, and gave her a 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 Bush administration may, if anything, have been less preoccupied by terrorism than the Clinton aides.

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 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 a 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 Qaeda threat to "swatting at flies." He said he wanted a plan to "bring this guy down."

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 involve the United States too deeply in a civil war and anger Pakistan.

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. (However, the leader of the Northern Alliance was assassinated by Qaeda agents on Sept. 9.) Mr. Bush was traveling on Sept. 10 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.