Zacarias Moussaoui was arrested in the United States in August after flight instructors grew suspicious because he wanted to learn how to steer a plane - but not how to land or take off.

Questions are now being asked about why his arrest did not lead US investigators to uncover the 11 September hijack plot.

French security sources say they told the Federal Bureau of Investigation (FBI) of his suspected links with extremist groups - and that he had spent time in Afghanistan.

Immigration charges

FBI director Robert Mueller said the authorities had decided in August that they lacked legal authority to search Mr Moussaoui's computer.

He was then being held on immigration charges.

It was later found to contain information on flight training and crop spraying from planes.

His inquiries about hiring a crop duster plane are now being seen as possible evidence of a plot to spread chemical, biological or radioactive material in an American or western city.

French tip-offs 'ignored'

Security sources in Paris say British police turned down French requests to question him in the 1990s - when he espoused radical Islam as a student in London.