I have said before that if I were a terrorist wanting to hit US aviation I would do it via an internal flight

David Learmount, Flight International

But security experts say the terrorists had been presented with virtually an open goal.

Security on US domestic flights is so relaxed that organised, determined extremists would have had few problems breaching it.

Key parts of international procedure - such as making sure bags are accompanied, and X-raying hold baggage - are absent.

The culture, say critics, puts passengers' freedom to travel efficiently above the need for watertight security.

"I have said before that if I were a terrorist wanting to hit US aviation I would do it via an internal flight," said David Learmount, Operations and Safety Editor of Flight International.

"I am not staggered by this. It's very simple - the US has high security on international flights, and virtually no security on internal flights.

Frankly as one travels around America on internal flights, one can see only too well that the Americans don't take security in their airports as seriously as we do in this country

Security expert Daniel Plesch

"It seems appalling that on a single day so many people could get through."

Another security expert, Daniel Plesch, told the BBC the Americans had simply not believed this could happen to them.

"Frankly as one travels around America on internal flights, one can see only too well that the Americans don't take security in their airports as seriously as we do in this country," he said.

"And at conferences in America, I have seen many security lapses which would make your hair stand on end.

"There isn't the same culture there as is in Israel or Britain to deal with these kind of threats. America is just too open."

Top-level attempts have been made to boost security.

UK has tougher airport security because of known threats

Within the last decade, a major commission headed by then US Vice-President Al Gore recommended increasing security to international levels - but the industry opposed the idea so strongly that the plan was never adopted, say industry insiders.

One expert called that "one of the most astounding decisions ever taken".

Now, with the appalling events of Tuesday, security is likely to be ratcheted up to its highest-ever level.

No-one knows what weapons the extremists used, but it was clearly enough to take over four separate planes.

'Sophisticated'

But even with much tougher security, an attack as staggering as this in its ambition and planning might still have succeeded.

Phil Butterworth-Hayes, the civil aviation editor with the Janes information group, says it would have been difficult to prevent such a sophisticated attack.

Aviation security tends to be retrospective. New measures are only put in place after something has happened

Phil Butterworth-Hayes, Janes information group

Terrorists are "always one step ahead" of the institutions that really need protection, he said.

"When you get people determined to commit acts of terrorism, it is almost impossible to stop them," he said.

"Aviation security tends to be retrospective. New measures are only put in place after something has happened.

'Most audacious attack'

"Aviation thought that it had sorted out the bombs-in-holds problem after Lockerbie, but now there is a fresh problem to resolve."

Chris Yates, aviation security editor of Jane's Defence Weekly, said the attack would have been way beyond the reach of ordinary security measures.

"This is perhaps the most audacious terrorist attack that's ever taken place in the world," he said.

"It takes a logistics operation from the terror group involved that is second to none."