The terrorists who crashed planes into the Pentagon and World Trade Centre probably were able to overcome the flight crews and then fly the airliners themselves, aviation safety experts suggested.

'It's just incredible that you have these four apparent breaches of security,' said Congressman John Mica, chairman of the House aviation subcommittee.

'We've seen from today that a determined terrorist isn't going to be stopped by a metal detector and a couple of quick questions about who packed their luggage,' he said.

Attorney General John Ashcroft said American Airlines Flight 11 that left Boston's Logan airport for Los Angeles 'was hijacked by suspects armed with knives.'

Television commentator Barbara Olson told her husband by mobile phone minutes before her flight was forced to crash into the Pentagon that attackers had used knifelike instruments to take over the plane.

Current airport security systems are designed to catch people carrying metal weapons such as guns and knives, said David Stempler of the Air Travellers Association. And in recent years, much effort also has been expended on developing devices to sniff out bombs.

Darryl Jenkins, director of George Washington University's Aviation Institute, agreed that the easiest way to hijack a plane is to board it without weapons.

'One thing about terrorists is just how flexible they are,' Jenkins said. 'When you put up a roadblock in one place, they go around and find other means.

'I'm a pilot, he added. 'None of us would ever fly a plane into the Trade Centre. We would take that bullet first. Terrorists flew the plane instead.'

That view was shared by Jim Burnett, a former chairman of the National Transportation Safety Board, who said a commercial pilot, 'even under duress, would not do that. It would have taken some skill on the part of whoever was able to take over the plane.'

Gene Poteat, president of the Association of Former Intelligence Officers, added: 'They flew the planes themselves. No pilot, even with a gun to his head, is going to fly into the World Towers.'

Stempler said the possibility of terrorists actually taking over planes and then flying them into targets is not one that he had ever heard discussed.

A radar track of American Airlines Flight 11 that struck the World Trade Centre showed that it left Boston en route to Los Angeles and began its path westward normally, but then made a sharp left turn to fly down the Hudson River to New York.

It was not known whether the pilot reported a hijacking. Even if a terrorist were known to be in control of a plane heading for a major city, coping would pose a huge challenge, David Stempler of the Air Travellers Association said.

'I don't think we are that primed and ready at this point. I don't think we could get the interceptors up fast enough to manage that,' he said.

Burnett said criminal investigations will be launched quickly to see whether there is a common thread in the apparent breaches of security that allowed terrorists aboard the planes.

If there is, it is likely to be plugged quickly, he said. 'However, I think it's safe to say that with the level of sophistication, there may be additional types of breaches that have been identified.'

Burnett said these terrorist acts will likely spur intense security and there could be substantial disruption of transportation for a time.

Poteat said the hijackings will lead to much tighter security on planes, including the possibility of putting armed marshals on planes again.

Armed marshals were used on planes a few decades ago after a series of hijackings to Cuba.

Kathleen Flynn, who lost her son in the bombing of Pan Am Flight 103 over Lockerbie in 1988 said: 'How in God's name were they able to hijack that many aircraft? Where was the security? I really want to know. It's going to change how America lives, we can never become this vulnerable again.'