In one of the most tantalising documents, they also hint at Iranian involvement in al-Qaeda suicide bombing.

Whether the Shia Islamic Republic has offered support to the Sunni militant group has been one of the most controversial questions in the Middle East.

American leaders have both claimed the existence of such links and backed away from them.

But a threat report in the files dated Nov 17, 2006 claims that new techniques for suicide bombing, a favoured al-Qaeda and Sunni insurgent practice in Iraq, had “surfaced” in Iran and Syria.

Both involved the use of miniature cameras to allow remote monitoring of the attack.