US aviation officials should have done more to protect passengers from suicide attacks, an independent commission into the 11 September 2001 attacks says.
The panel is probing national security lapses ahead of the attacks
It also said the government missed warning signs that could have stopped some of the hijackers.
The Commission on Terrorist Attacks upon the United States was issuing a preliminary report on its findings.
The commission says it has asked for extra time to compile its final report,
which had been due in May.
In an emotionally-charged session, the commission praised the actions of flight staff and played a four-minute recording of Betty Ong from American Airlines Flight 11 which was crashed into the World Trade Center.
"Our Number One got stabbed, our purser is stabbed. Nobody knows who stabbed who, and we can't even get up to business class right now because nobody can breathe," she was heard to say.
The preliminary report found that aviation security officials had looked at the possibility of suicide hijackings as far back as March 1998 - nearly six years ago.
But it said they concentrated on another threat they considered more likely - explosives being smuggled on board commercial airliners.
The chairman of the independent commission, Thomas Kean, said this made it easier for the hijackers to carry out the 11 September attacks.
"We learned that the pre-screening process selected nine of the 19 hijackers, but that the consequences of that selection only meant matching baggage to passengers to prevent them from bringing explosives on the planes.
"Therefore the pre-screening was irrelevant to defeating this particular plot. We learned that deadly knives with blades less than four inches long could be bought on board aircraft and apparently were," Mr Kean said.
Commission vice-chairman Lee Hamilton said opportunities to stop the hijackers had been missed repeatedly, including suspicious indicators like:
- fraudulent passports
- false statements made on the visa application
- violations of the immigration law
"You go through all of this and you see that we had and lost a lot of opportunities to stop these people," Mr Hamilton said.
The commission said the hijackers were also able to carry out the attacks because airline staff were trained not to resist hijackers.
But it praised the actions of staff, like 45-year-old Ong, a flight attendant who made a phone call shortly before her death.
"We can't even get into the cockpit. We don't know who's
there," Ong said, before the call ended in a dialling tone.
The commission is supposed to issue a final report by May - but has called for this deadline to be extended to July - so it can present a complete report on what lessons can be learnt from the events of 11 September 2001.
But officials in the White House are concerned releasing the report in July could impinge on President Bush's re-election campaign which will be well underway by then.