A terrorism expert said he believed the pilots of the four hijacked aircraft used in the attacks in America yesterday were probably murdered before the crashes.
Mr Gene Poteat, president of the US Association of Former Intelligence Officers, said "no pilot, even with a gun to his head, would fly into the World Towers".
Mr Poteat added the hijackers may also have learned how to disable communications systems used to alert authorities to trouble.
He estimated at least 30 highly-trained operatives would have been needed to synchronise the attacks.
Experts expressed disbelief at how no fewer than four planes were able to be hijacked at the same time.
Mr Kieran Daly, editor of the Air Transport Intelligence internet news service stated: "People might have infiltrated into aviation systems years ago and then merely waited to ensure that all the attacks worked."
Mr Steven Emerson of the US-based Investigative Project, a research group focused on international terrorism said it was a scheme that defied all scenarios examined by US security officials.
AP