WASHINGTON, April 17 - Early this year, the independent commission investigating the Sept. 11 attacks played four minutes of a call from Betty Ong, a crew member on American Airlines Flight 11. The power of her call could not have been plainer: in a calm voice, Ms. Ong told her supervisors about the hijacking, the weapons the attackers had used, the locations of their seats.

At first, however, Ms. Ong's reports were greeted skeptically by some officials on the ground. "They did not believe her," said Bob Kerrey, a commission member. "They said, `Are you sure?' They asked her to confirm that it wasn't air-rage. Our people on the ground were not prepared for a hijacking."

For most Americans, the disbelief was the same. The attacks of Sept. 11 seemed to come in a stunning burst from nowhere. But now, after three weeks of extraordinary public hearings and a dozen detailed reports, the lengthy documentary record makes clear that predictions of an attack by Al Qaeda had been communicated directly to the highest levels of the government.

The threat reports were more clear, urgent and persistent than was previously known. Some focused on Al Qaeda's plans to use commercial aircraft as weapons. Others stated that Osama bin Laden was intent on striking on United States soil. Many were passed to the Federal Aviation Administration.

While some of the intelligence went back years, other warnings - including one that Al Qaeda seemed interested in hijacking a plane inside this country - had been delivered to the president on Aug. 6, 2001, just a month before the attacks.

The new information produced by the commission so far has led 6 of its 10 members to say or suggest that the attacks could have been prevented, though there is no consensus on when, how or by whom. The commission's chairman, Thomas H. Kean, a Republican, has described failures at every level of government, any of which, if avoided, could have altered the outcome. Mr. Kerrey, a Democrat, said, "My conclusion is that it could have been prevented. That was not my conclusion when I went on the commission."

While the commission was created to diagnose mistakes and to recommend reforms, its examination has powerful political resonance. The panel has reviewed the records of two presidents, Bill Clinton and George W. Bush.

Mr. Bush, who is in the midst of a campaign for re-election, said last Sunday that none of the warnings gave any hint of the time, place or date of an assault. "Had I known there was going to be an attack on America I would have moved mountains to stop the attack," he said.

In an intense stretch this month, the commission pried open some of the most closely guarded compartments of government, revealing the flow and details of previously classified information given to two presidents and their senior advisers, and the performance of intelligence and law enforcement officials.

The inquiry has gone beyond the report of a joint panel of the House and Senate intelligence committee in 2002, which chronicled missteps at the mid-level of bureaucracies. Urged on by a number of families of people killed in the attacks, the Kean commission has used a mix of moral and political leverage to extract presidential communications and testimony. Among the new themes that have fundamentally reshaped the story of the Sept. 11 attacks are:

¶Al Qaeda and its leader, Mr. bin Laden, did not blindside the United States, but were a threat recognized and discussed regularly at the highest levels of government for nearly five years before the attacks, in thousands of reports, often accompanied by urgent warnings from lower-level experts.

¶Presidents Clinton and Bush received regular information about the threat of Al Qaeda and the intention of the bin Laden network to strike inside the United States. Each president made terrorism a stated priority, failed to find a diplomatic solution and viewed military force as a last resort. At the same time, neither grappled with the structural flaws and paralyzing dysfunction that undermined the C.I.A. and the F.B.I., the two agencies on which the nation depended for protection from terrorists. By the end of his second term, Mr. Clinton and the director of the F.B.I., Louis J. Freeh, were barely speaking.

¶Even when the two agencies cooperated, the results were unimpressive. Mr. Kean said that he viewed the reports on the two agencies as indictments. In late August 2001, George J. Tenet, the director of central intelligence, learned that the F.B.I. had arrested Zacarias Moussaoui after he had enrolled in a flight school. Mr. Tenet was given a memorandum titled "Islamic Extremist Learns to Fly." But he testified that he took no action and did not tell President Bush about the case.

During the Clinton years, particularly at the National Security Council, the commission has found, there was uncertainty about whether the threat posed by Al Qaeda and Mr. bin Laden justified military action. Much of the debate was provoked by Richard A. Clarke, who led antiterrorism efforts under both Mr. Clinton and Mr. Bush and argued for aggressive action.

"Former officials, including an N.S.C. staffer working for Mr. Clarke, told us the threat was seen as one that could cause hundreds of casualties, not thousands," according to one interim commission report. "Such differences affect calculations about whether or how to go to war. Even officials who acknowledge a vital threat intellectually may not be ready to act upon such beliefs at great cost or at high risk."

In the first eight months of the Bush administration, the commission found, the president and his advisers received far more information, much of it dire in tone and detailed in content, than had been generally understood.

The most striking came in the Aug. 6 memorandum presented in an intelligence briefing the White House says Mr. Bush requested. Titled "Bin Laden Determined to Strike in U.S.," the memorandum was declassified this month under pressure from the commission. After referring to a British tip in 1998 that Islamic fundamentalists wanted to hijack a plane, it went on to warn: "Nevertheless, F.B.I. information since that time indicates patterns of suspicious activity in this country consistent with preparations for hijackings or other types of attacks." Mr. Bush has said the briefing did not provide specific details of when and where an attack might take place.

Mr. Kerrey said that Mr. Bush showed "good instincts" by asking for the material, but said the call from Ms. Ong, the flight attendant on American Airlines Flight 11 - which crashed into the north tower of the World Trade Center in the day's first attack - showed that the threats and alarms did not get passed down the line.

"I don't see any evidence that our airports were on heightened alert," he said. "A hijacking was not a bolt out of the blue."

The Clinton Response: A Growing Priority, Hamstrung by Process

Throughout President Clinton's eight years in office, law enforcement and intelligence agencies tracked Al Qaeda through a succession of plots in the United States and overseas. The commission found new evidence that counterterrorism became a priority for the Clinton national security team. But the panel said the effort was stymied by bureaucratic miscommunications, diplomatic failures, intelligence lapses and policy miscalculations.

On the intelligence side, the commission discovered confusion about crucial issues. White House aides believed, for example, that President Clinton had authorized actions to kill Mr. bin Laden, but C.I.A. officers thought they were legally permitted to kill him only during an attempt to capture him.

Throughout the 1990's, the panel found, law enforcement and intelligence experts, often in lower-level jobs, repeatedly warned that Mr. bin Laden wanted to strike inside the United States. The threat was plainly stated in documents disclosed by the commission. One, in 1998, was titled "Bin Laden Threatening to Attack U.S. Aircraft," and cited the possibility of a strike using antiaircraft missiles. Another 1998 report, referring to Mr. bin Laden as "UBL," said, "UBL Plans for Reprisals Against U.S. Targets, Possibly in U.S." A 1996 review of a plot to blow up airliners over the Pacific uncovered evidence of the Qaeda interest in crashing a hijacked plane into C.I.A. headquarters in Langley, Va.

But the C.I.A.'s efforts to thwart Mr. bin Laden's network through covert action were ineffectual, the commission found. The agency's "Issue Station," which was set up in 1996 to hunt down Mr. bin Laden, had a half-dozen chances to attack the Qaeda chief, but each time agency higher-ups balked. A plan to kill him in February 1999 was called off at the last minute because of concerns that he might be with a prince from the United Arab Emirates, regarded as a useful ally in counterterrorism, the commission reported.

President Clinton tried diplomacy, but that too failed. In 1998, Mr. bin Laden issued a public call for any Muslim to kill any American anywhere in the world. That April, Bill Richardson, the United States representative to the United Nations, went to Afghanistan and asked the Taliban government to surrender Mr. bin Laden to the United States.

Simultaneous Qaeda bombings in August 1998 at American Embassies in Kenya and Tanzania galvanized talk of aggressive efforts, but brought no tangible results. President Clinton ordered cruise missile strikes against a terrorist training camp in Afghanistan and a suspected chemical weapons plant in the Sudan. The missiles hit their intended targets, but neither Mr. bin Laden nor any other terrorist leader was killed.

In December 1998, Mr. Tenet announced in a memorandm to his senior staff at the C.I.A. that they would henceforth be at war with Al Qaeda. "I want no resources or people spared," he wrote.

In practice, the commission concluded, Mr. Tenet's declaration of war, which the C.I.A. director has frequently cited in his public testimony since the attacks, had "little overall effect."

The Federal Bureau of Investigation, the country's other principal counterterrorism agency, struggled to repackage the tools of an interstate crime-fighting organization against a highly unconventional foreign-based threat to the United States.

One interim panel report described the F.B.I. as a bureaucracy suffocated by outmoded rules and legal barriers that barred criminal investigators from obtaining intelligence data. Agents worked on an aging computer system that kept them from knowing what other agents in their own offices, much less those around the country, were working on. Some F.B.I. analysts hired to assess terror threats were assigned to jobs entering data and answering telephones.

Throughout the 1990's, the bureau focused on investigations of specific terror attacks to bring criminal cases to court. The most successful were handled by its New York office, whose agents were among the most knowledgeable in the world about Al Qaeda.

By late in the decade, the F.B.I. recognized the need to improve its intelligence collection and analysis, but the report said that Mr. Freeh had difficulty reconciling that with its continuing agenda, including the war on drugs. As a result, the bureau's counterterrorism staff was thin. On Sept. 11, 2001, only about 6 percent of the F.B.I.'s agent work force was assigned to terrorism.

In October 2000, two Qaeda suicide bombers in a small boat packed with explosives attacked the Navy destroyer Cole in the Yemeni port of Aden, killing 17 American sailors. President Clinton did not retaliate, but Samuel R. Berger, Mr. Clinton's national security adviser, warned his successor, Condoleezza Rice, that "she would be spending more time on terrorism and Al Qaeda than any other issue."

The Bush Review: Alerts, but Breaks in Chain of Command

Warned of the Qaeda threat during the transition, President Bush's national security team started work in March 2001 on a comprehensive strategy to eradicate the terror network. But the effort seemed to plod ahead almost in isolation from the urgent notices by the C.I.A. Most of the threat warnings, but not all, pointed overseas.

At the end of May, Cofer Black, chief of the C.I.A.'s counterterrorism center, told Ms. Rice that the threat level stood at "7 on a scale of 10, as compared to an 8 during the millennium," the period around January 2000. In response, American embassies were warned to take precautions. The State Department warned Americans traveling overseas. The C.I.A. intensified operations to disrupt terror cells around the world.

Mr. Tenet took his terror warnings directly to Mr. Bush. Ms. Rice said that at least 40 meetings between the C.I.A. director and the president dealt "in one way or other with Al Qaeda or the Al Qaeda threat." Mr. Tenet later said "the system was blinking red," adding that no warning indicated that terrorists would fly hijacked commercial aircraft into buildings in the United States.

On July 5, Ms. Rice and Andrew H. Card Jr., the White House chief of staff, asked Mr. Clarke to alert top officials of the country's domestic agencies. "Let's make sure they're buttoning down," Ms. Rice said. The F.A.A. issued threat advisories, but neither the agency's top administrator nor Norman Y. Mineta, the secretary of transportation, was aware of the increased threat level, said Jamie S. Gorelick, a commission member, at a hearing last week.

On July 27, Mr. Clarke informed Ms. Rice that the threat reporting had dropped. But White House officials said that Mr. Bush continued to ask about any evidence of a domestic attack. In August, C.I.A. officials prepared a briefing about the possibility of Qaeda operations inside the United States, including the use of aircraft in terror attacks.

The briefing paper was presented to Mr. Bush on Aug. 6 at his Texas ranch. The memorandum, declassified on April 10 by the White House at the commission's request, included some ominous information. It said that Qaeda operatives had been in the United States for years, might be planning an attack in the United States and could be focusing on a building in Lower Manhattan as a target.

Mr. Bush said the Aug. 6 report was not specific enough to order new actions. "I am satisfied that I never saw any intelligence that indicated there was going to be an attack on America at a time and place, an attack. Of course I knew that America was hated by Osama bin Laden. That was obvious. The question was, who was going to attack us, when and where and with what?"

The president noted that the memo said the F.B.I. had 70 investigations under way related to Al Qaeda. In addition, the F.B.I. had sent messages to its field offices urging agents to be vigilant. Thomas J. Pickard, the F.B.I.'s acting director from June to August, said he telephoned top agents to advise them of the threat. But the commission found that most F.B.I. personnel "did not recall a heightened sense of threat from Al Qaeda."

The commission found several previously undisclosed intelligence reports to Mr. Bush, Vice President Dick Cheney and national security aides dating back to April and May, when the volume of warnings began to increase. Mr. Bush was given briefing papers headlined, "Bin Laden Planning Multiple Operations," "Bin Laden Threats Are Real" and "Bin Laden's Plans Advancing."

In August 2001, the F.B.I. and the C.I.A. came as close as the government ever did to detecting anyone connected to the Sept. 11 plot. That month investigators finally made progress in the fractured effort to track down two men, Khalid al-Midhar and Nawaq Alhazmi, who on Sept. 11 were aboard American Airlines Flight 77, which crashed into the Pentagon.

The C.I.A. had investigated the pair off and on since they had been seen at a Qaeda meeting in Malaysia in January 2000. But they were not placed on a State Department watch list until Aug. 23, after they already were in the United States. Moreover, the C.I.A. failed to tell the F.B.I.'s primary investigators on the Cole case of a key connection between the two men and a Cole suspect until after Sept. 11. "No one apparently felt they needed to inform higher level of management in either the F.B.I. or C.I.A. about the case," one commission report said.

In mid-August, after the arrest of Mr. Moussaoui in Minneapolis, the commission disclosed, Mr. Tenet and his top deputies were sent a briefing paper labeled "Islamic Extremist Learns to Fly." But they took no action on the report.

The commission found several missed opportunities in the Moussaoui investigation that might have detected his connection to a Qaeda cell in Hamburg, Germany, that planned the Sept. 11 attacks. "A maximum U.S. effort to investigate Moussaoui could conceivably have unearthed his connections to the Hamburg cell," one commission report said. The report added that publicity about Mr. Moussaoui's arrest "might have disrupted the plot. But such an effort would have been a race against time."

It was not until Sept. 10 that Mr. Bush's national security aides approved a three-phase strategy to eliminate Al Qaeda. The plan, which was to unfold over three to five years, envisioned a mission to the Taliban in Afghanistan, where Al Qaeda was based; increased diplomatic pressure; and covert action. Military strikes might be used, but only if all other means failed.

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