The report discloses that the Federal Aviation Administration, despite being focused on risks of hijackings overseas, warned airports in the spring of 2001 that if "the intent of the hijacker is not to exchange hostages for prisoners, but to commit suicide in a spectacular explosion, a domestic hijacking would probably be preferable."

The report takes the F.A.A. to task for failing to pursue domestic security measures that could conceivably have altered the events of Sept. 11, 2001, like toughening airport screening procedures for weapons or expanding the use of on-flight air marshals. The report, completed last August, said officials appeared more concerned with reducing airline congestion, lessening delays, and easing airlines' financial woes than deterring a terrorist attack.

The Bush administration has blocked the public release of the full, classified version of the report for more than five months, officials said, much to the frustration of former commission members who say it provides a critical understanding of the failures of the civil aviation system. The administration provided both the classified report and a declassified, 120-page version to the National Archives two weeks ago and, even with heavy redactions in some areas, the declassified version provides the firmest evidence to date about the warnings that aviation officials received concerning the threat of an attack on airliners and the failure to take steps to deter it.

Among other things, the report says that leaders of the F.A.A. received 52 intelligence reports from their security branch that mentioned Mr. bin Laden or Al Qaeda from April to Sept. 10, 2001. That represented half of all the intelligence summaries in that time.

Five of the intelligence reports specifically mentioned Al Qaeda's training or capability to conduct hijackings, the report said. Two mentioned suicide operations, although not connected to aviation, the report said.

A spokeswoman for the F.A.A., the agency that bears the brunt of the commission's criticism, said Wednesday that the agency was well aware of the threat posed by terrorists before Sept. 11 and took substantive steps to counter it, including the expanded use of explosives detection units.

"We had a lot of information about threats," said the spokeswoman, Laura J. Brown. "But we didn't have specific information about means or methods that would have enabled us to tailor any countermeasures."

She added: "After 9/11, the F.A..A. and the entire aviation community took bold steps to improve aviation security, such as fortifying cockpit doors on 6,000 airplanes, and those steps took hundreds of millions of dollars to implement."

The report, like previous commission documents, finds no evidence that the government had specific warning of a domestic attack and says that the aviation industry considered the hijacking threat to be more worrisome overseas.

"The fact that the civil aviation system seems to have been lulled into a false sense of security is striking not only because of what happened on 9/11 but also in light of the intelligence assessments, including those conducted by the F.A.A.'s own security branch, that raised alarms about the growing terrorist threat to civil aviation throughout the 1990's and into the new century," the report said.

In its previous findings, including a final report last July that became a best-selling book, the 9/11 commission detailed the harrowing events aboard the four hijacked flights that crashed on Sept. 11 and the communications problems between civil aviation and military officials that hampered the response. But the new report goes further in revealing the scope and depth of intelligence collected by federal aviation officials about the threat of a terrorist attack.

The F.A.A. "had indeed considered the possibility that terrorists would hijack a plane and use it as a weapon," and in 2001 it distributed a CD-ROM presentation to airlines and airports that cited the possibility of a suicide hijacking, the report said. Previous commission documents have quoted the CD's reassurance that "fortunately, we have no indication that any group is currently thinking in that direction."

Aviation officials amassed so much information about the growing threat posed by terrorists that they conducted classified briefings in mid-2001 for security officials at 19 of the nation's busiest airports to warn of the threat posed in particular by Mr. bin Laden, the report said.

Still, the 9/11 commission concluded that aviation officials did not direct adequate resources or attention to the problem.

"Throughout 2001, the senior leadership of the F.A.A. was focused on congestion and delays within the system and the ever-present issue of safety, but they were not as focused on security," the report said.

The F.A.A. did not see a need to increase the air marshal ranks because hijackings were seen as an overseas threat, and one aviation official told the commission said that airlines did not want to give up revenues by providing free seats to marshals.

The F.A.A. also made no concerted effort to expand their list of terror suspects, which included a dozen names on Sept. 11, the report said. The former head of the F.A.A.'s civil aviation security branch said he was not aware of the government's main watch list, called Tipoff, which included the names of two hijackers who were living in the San Diego area, the report said.

Nor was there evidence that a senior F.A.A. working group on security had ever met in 2001 to discuss "the high threat period that summer," the report said.

Jane F. Garvey, the F.A.A. administrator at the time, told the commission "that she was aware of the heightened threat during the summer of 2001," the report said. But several other senior agency officials "were basically unaware of the threat," as were senior airline operations officials and veteran pilots, the report said.

The classified version of the commission report quotes extensively from circulars prepared by the F.A.A. about the threat of terrorism, but many of those references have been blacked out in the declassified version, officials said.

Several former commissioners and staff members said they were upset and disappointed by the administration's refusal to release the full report publicly.

"Our intention was to make as much information available to the public as soon as possible," said Richard Ben-Veniste, a former Sept. 11 commission member.