Those privy to the intelligence community's classified reports on domestic terrorism had plenty of reason to lose sleep in the spring and summer of 2001. Analysts warned of potential attacks by unspecified terrorists in New York and California, and by operatives of Osama bin Laden somewhere in the United States.

CIA sources in Afghanistan picked up chatter about an unspecified, impending attack, and the National Security Agency monitored at least 33 communications suggesting an imminent attack, according to a congressional investigative report issued yesterday. Bin Laden operatives were dropping out of sight, and according to some classified warnings, headed for the United States and Canada.

The classified alarms reached a crescendo at the beginning of July, when top U.S. officials were warned that bin Laden was in the throes of advanced preparations for a major attack, most likely against an American or Israeli target. "The attack will be spectacular and designed to inflict mass casualties against U.S. facilities or interests," they were told.

But in the critical month of August, the government's complex and balky counterterrorism machinery failed to move fast enough to stop accelerating preparations for an attack.

A Frenchman of Arab descent was detained in Minnesota after his flight training aroused suspicion; the CIA posted a request for detention of two Saudi men with worrisome al Qaeda connections, both already in the United States.

But U.S. officials did not develop a strategy able to match a peril that only some analysts believed was grave and immediate, according to the historical record in the joint report by the House and Senate intelligence committees. Because of now familiar shortcomings in internal cooperation and communication, the U.S. government never came close to disrupting the Sept. 11 attacks.

It was not, the committees said, for lack of opportunity. Instead, scattered moments of superb intelligence-gathering and analysis were either not noticed or not heeded. And no one pulled together disparate bits of information from immigration, intelligence, aviation and other federal records into a comprehensive and actionable tableau of motive, plot, preparation and imminent danger.

The well-documented ability of the hijackers to stay many steps ahead of their disorganized U.S. government foes is, after all, the principal reason why almost all the intelligence agencies implicated in the disaster have been at least partly reorganized and why the Department of Homeland Security was created from 21 agencies with 180,000 employees.

But the report -- and particularly its detailed chronology of events in the last weeks when the attack might have been prevented -- makes clear that the disaster was the result as much of lapses in government follow-through as it was the result of defects of intelligence.

For example, the July 2001 "Phoenix" memo, written by an FBI agent in Arizona, warned about "an inordinate number of individuals of investigative interest" taking flight training. It urged the agency to collect data on flight schools and foreign students, and to discuss the potential threat with other intelligence agencies. The memo appears especially prescient now, because one of the men mentioned in the memo was arrested in Pakistan in 2002 with a senior al Qaeda facilitator, Abu Zubayda.

But the memo was not shared with the CIA, aviation or immigration authorities. Moreover, it "generated little or no interest" among the top FBI officials in Washington and New York who received it, the committees concluded. In their report, the lawmakers decried that "the best example of . . . creative, imaginative, and aggressive analysis of relevant intelligence" they saw during the probe never got any traction.

August was the month that senior government officials were warned in a widely circulated classified report that al Qaeda apparently had a support structure in the United States. It was also the month that a document specifically written for the president warned of bin Laden's long-held desire to conduct attacks within the United States, and his possible preparations for airplane hijackings.

In mid-August, U.S. immigration agents detained Zacarias Moussaoui, a French national enrolled at a Minnesota flight school, for overstaying his visa. They acted after local FBI agents became suspicious that he was involved in a hijacking plot, but decided -- based on what the agency later determined was an erroneous interpretation of the law -- that they could not search his belongings.

In those belongings, the committee's report said, were papers linking him to suspected al Qaeda operatives.

On Aug. 21, the Minneapolis FBI field office sent an e-mail to headquarters in Washington, saying that the U.S. Secret Service should be warned about Moussaoui's threat potential. If he seized an aircraft flying from London to New York, "it will have the fuel on board to reach DC," the e-mail said. But the committees found that the headquarters' message, in response to the e-mail and eventually issued on Sept. 4, did "not provide any analysis of Moussaoui's actions or plans," put his actions in context, or even describe what threat he posed.

Even the government's anti-terrorism agencies were recognizing their own failures. On the day of Moussaoui's detention, a CIA Counter-Terrorism Center report had concluded that "for every UBL [Usama Bin Laden] that we stop, an estimated 50 operatives slip thought our loose net undetected. . . . It is clear that UBL is building up a worldwide infrastructure which will allow him to launch multiple and simultaneous attacks with little or no warning."

Nawaf Alhazmi and Khalid Almihdhar -- who would become two of the principal hijackers -- were perfect illustrations. Although Alhazmi had been in the country for more than a year and Almihdhar arrived in New York on July 4, poor communication between the CIA and the FBI left key authorities ignorant of their August whereabouts and their links to an al Qaeda operative implicated in the bombing of the USS Cole, a Navy vessel at port in Yemen.

The committees ticked off the mistakes: The CIA had designated both as "terrorist operatives" in January 2000 and warned that Alhazmi's travel may be in support of terrorist missions. Almihdhar had been the focus of an intensive CIA search in Thailand that year, but not in the United States. A CIA directive in December 1999 had demanded that "terrorist personality information" be flagged for the State Department and others. But a CIA officer told the committee he decided not to tell an FBI agent about Almihdhar's U.S. visa at a key meeting in June because the information would "not mean anything" to the agent.

Finally, on Aug. 23, the CIA sent a notice to the State Department, Customs and Immigration Services -- but not to the Federal Aviation Administration -- requesting that Almihdhar and Alhazmi be added to their watch lists and formally denied entry to the United States. They knew that Almihdhar was already here, but sought to ensure he would be stopped if he tried to leave. The State Department acted the next day, and then revoked Almihdhar's visa two days later.

But in a critical omission, officials at the Treasury Department's Financial Crimes Enforcement Network and the FBI's Financial Review Group -- which both have the power to tap into private credit card and bank data -- were not informed, the committees said. Officials of both groups said later that had they been told, they could readily have located Alhazmi and Almihdhar, who were using credit cards to supplement their income from funds sent from overseas.

The next day, CIA's Counter-Terrorism Center alerted stations around the world about Moussaoui. The FBI followed up with its own general alert about the arrest on Sept. 4, but failed to request any particular action or link him to a broader threat of an al Qaeda attack.

On Aug. 24, according to the committee, the hijackers began to purchase their airplane tickets. An FBI agent in New York requested a full criminal investigation of Almihdhar, but an unnamed FBI headquarters official rejected the request, citing the absence of conclusive information about Almihdhar's involvement in a substantial federal crime, and the need to maintain a "wall" between intelligence and any criminal probes.

The New York agent protested that "someday someone will die" because the FBI's reticence was protecting only bin Laden, not potential victims. But the protest had no effect, and no probe was launched; also, no FBI check was made on Alhazmi. The search was largely limited to checking to see if Almihdhar was registered at a New York Marriott hotel or had a driver's license.

Neither turned up any information, and so FBI agents in Los Angeles were asked to look for Almihdhar and check airline records, the committee said.

The new anti-bin Laden presidential directive was declared ready for the president's consideration on Sept. 10. That same day, the NSA obtained two intercepts quoting suspected terrorists predicting a significant development on Sept. 11. The intercepts were not translated until Sept. 12, the committee said in a heavily redacted passage of the report, "because of the nature of the processes involved."

On Sept. 11, Almihdhar and Alhazmi boarded American Airlines flight 77 without difficulty and hijacked it during its flight before crashing it into the Pentagon.

Although al Qaeda operative Khalid Almihdhar, left, arrived in the United States in July 2001 and fellow hijacker Nawaf Alhazmi had been in the country for more than a year, poor communications kept key U.S. authorities ignorant of their whereabouts before the Sept. 11 attacks.In undated video obtained by Kuwaiti newspaper in June 2001, al Qaeda's Osama bin Laden takes aim.