Unlike the eight 'lead' attackers, who were all trained pilots, they did not leave messages for friends and family indicating they knew their lives were over. None of them had copies of the instructions for prayer and contemplation on the eve of the attacks and for 'opening your chest to God' at the moment of immolation, which FBI agents discovered in the luggage of Mohamed Atta, the man believed to be the hijackers' leader, who flew the first plane to destruction in New York.

It is understood the FBI has found evidence suggesting the 11 men expected to take part in 'conventional' hijackings - with the planes flown to distant airports, and the passengers and crew taken hostage while the hijackers presented demands. Items found among the 11 men's possessions suggest they had been preparing themselves for incarceration. One source said: 'It looks as if they expected they might be going to prison, not paradise.'

The FBI analysis concludes the 11 may have believed the purpose of the hijackings was to free the perpetrators of previous extremist terrorist attacks on the United States, such as the first World Trade Centre bombing in 1993.

Other clues suggest the purpose for the 11 was to provide 'muscle': to overwhelm the passengers and crew. They had arrived in the US only recently and had not had pilot training.

Atta's final instructions, with their pleas for divine forgiveness, indicate that even the most fanatical fundamentalist had to make considerable psychological preparations before setting off to cause thousands of civilian deaths. Selecting those ready to carry out such a mission would not have been easy.

By keeping a majority of the hijackers in the dark as to their real purpose, these problems were avoided, the sources said.

Western intelligence services say the FBI's conclusions help to explain why, despite strong indications that Osama bin Laden's al-Qaida network was planning a spectacular atrocity, the West remained ignorant about its scale, location and detail.

'Of course it is inescapable that this was a terrible intelligence failure,' one Whitehall source said. 'But the FBI analysis at least puts it into context. The terrorists' security was extraordinarily tight. They were employing intelligence organisations' most basic principle: the need to know.'

At the same time, Western security chiefs say another suicide hijacking of a passenger aircraft would be far more difficult: assuming their fate to be death, passengers would probably deal swiftly with an attempt.

However, sources say they do fear other types of airborne attack, such as with hired executive jets. It is thought al-Qaida has up to 50 trained pilots who could mount attacks of this kind.

Meanwhile, it emerged last night that MI6 has advised the FBI to carry out blood tests of the numerous suspects now in US custody in connection with the hijackings to ascertain whether they have come into contact with biological terrorist materials.

The advice stems from MI6's experience in 1993, when Kanatjan Alibekov, the former head of the Soviet biological weapons research programme, defected to Britain. When Alibekov first approached the West in Paris, his bona fides were doubted. His claims to have worked on a variety of biological weapons were eventually verified by checking his blood for antibodies. Alibekov - who now lives in the US under the name Ken Alibek researching cures for life-threatening diseases - was found to be carrying antigens to all the agents he claimed to have used.

Exposure to even a small quantity of an agent such as anthrax - too small to cause symptoms - would leave antibodies in the blood.