The pilots of the doomed American airliners could have been dead by the time their planes hit their targets, say airline experts.

Ex-pilots believe that even a man with a gun to his head would not fly into a building housing thousands of people - and faced with certain death, would crash the plane elsewhere.

"It would be dead easy to aim an aircraft at a target that big," he said.

"They may have shot the pilots, or taken out at least one of them, and taken over the controls.

"I doubt the pilots were at the controls."

Other pilots also believe the crews were dead.

"I'm sure they would never have got an airline pilot to have done their dirty work. I hope it was mercifully quick," said one contributor to the pilots' internet site, the Professional Pilots' Rumour Network.

Nightmare scenario

Another pilot writes: "The thoughts that one must face when ordered to fly into a building are beyond my comprehension. One hopes/presumes the terrorists flew the last few minutes."

Before the mass hijacking, no-one had anticipated a scenario where extremists would turn not just themselves, but whole planes, into suicide bombs.

Not even Hollywood scriptwriters had dreamt up such a horrific chain of events.

Certainly no-one on the plane could possibly have realised at the start of the drama that the whole flight was doomed whatever anyone did.

Air traffic control personnel "would have known immediately" of a crisis when the planes veered away from their normal route, according to Chris Yates, aviation security editor of Jane's Defence Weekly.

"They are monitoring the flight paths all the time on screens in front of them," he said.

All conversations with the air traffic controllers will have been automatically recorded.

But the only record of all cockpit conversations - the planes' voice recorders - may not have survived, said Mr Learmount.

The boxes can withstand intense heat for only relatively short periods, and could have been destroyed in the infernos which followed the crashes.

As the drama began, the cockpit doors on all planes would have been locked - standard practice in the US, although not in the UK and Europe.

But the doors are not reinforced, and even without explosives, experts say it would be relatively easy to gain access - either by force or by forcing a cabin crew member to lead the way.

Once the hijackers were in place - and in control - Mr Yates said there would have been little anyone could do. If a plane, for whatever reason, is bound for destruction, controllers would have had to watch helplessly as tragedy unfolded on their screens.

"They will have been frantically trying to contact the planes that have gone off their trajectory," said Mr Yates.

Military option

"But if that doesn't work their only option would be possibly to scramble military jets."

But the military choices would have been limited by the circumstances anyway - with hundreds of passengers on board, and city centres underneath.

"This is sophisticated, suicidal and there's a degree of ruthlessness which we haven't ever seen in the use of terrorism anywhere in the world before," he said.

"This is quite unprecedented, it's on a different scale and it does require detailed knowledge and organisation, and a degree or sophistication which we would be surprised at, I think."

Investigators must now plough through tonnes of rubble to try finding the so-called "black boxes", along with hundreds of bodies.

The task may prove fruitless.

"We may never know what happened," said Mr Learmount.