America's aviation authority received numerous warnings about al-Qaeda attacks in the six months before 9/11, including five that mentioned hijackings and two that mentioned suicide operations, it has emerged.

A previously published report by the commission set up to investigate the September 11 attacks on the United States reveals that the US Federal Aviation Authority received 52 intelligence reports on al-Qaeda between April and September 2001.

The 911 Commission report criticises the FAA for failing to strengthen security measures in light of the reports, and accuses it of allowing a false sense of security to prevail.

The aviation industry was more concerned with hijacking threats overseas and did not appear to give serious credit to the idea of hijackings at home, the report added.

"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 FAA's own security branch, that raised alarms about the growing terrorist threat to civil aviation," the report said.

The striking findings of the staff report, dated August 2004, emerged after an investigation by The New York Times.

The full version of the report was blocked from public release by the White House for more than five months. A declassified and heavily edited version was filed in the National Archives two weeks ago.

So many reports on Osama bin Laden or his al-Qaeda terrorist network were received by the FAA in the months before 9/11, that between March and May it conducted classified briefings on the al-Qaeda threat for security officials at 19 of the busiest airports in the United States.

The report said that the 52 warnings on al-Qaeda sent to the FAA in that period amounted to half of all the intelligence reports it received.

The FAA did issue an alert to airports and airlines in early 2001, in the form of a CD-ROM, which mentioned the possibility that terrorists might conduct suicide hijackings.

But it said that a domestic hijacking would be difficult, and "fortunately we have no indication that any group is currently thinking in that direction."

The CD-ROM briefings said: "We don't rule it out ... If however, 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 9/11 panel said that this showed that the FAA had indeed considered the possibility that terrorists would hijack a plane and use it as a weapon, but had failed to direct adequate resources or attention to the problem.

"Throughout 2001, the senior leadership of the FAA 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.

FAA spokeswoman Laura Brown defended the authority, saying that the warnings were not specific enough to prevent the attacks. She said that the FAA did take steps to counter the terrorist threat, such as placing more explosives detection units in airports.

"We had a lot of information about threats," she said.

"But we didn’t have specific information about means or methods that would have enabled us to tailor any countermeasures."

Publication of the report has revived suspicions that the Bush administration tried to censor information that pointed to lax security before 9/11.

US Representatives Henry Waxman and Carolyn Maloney have requested House hearings on "whether the administration misused the classification process to withhold, for political reasons, official 9/11 Commission staff findings detailing how federal aviation officials received multiple intelligence reports warning of airline hijackings and suicide attacks before September 11," and "on the veracity of statements, briefings and testimony by then national security advisor Condoleezza Rice", they said in a statement.

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