Officers were drawing up “I2 Anacapa” charts — specialist diagrams used to build up a coherent picture from the flood of information produced by an investigation. Scores of officers had been diverted from normal duties on to the investigation into the London bombings.

“People were buzzing around working flat out. It was very impressive,” said an official who visited the centre.

As the service reacted to the atrocity that killed at least 55 people, it contacted all its field agents for any hint of who the attackers were, any information on recent sightings of Al-Qaeda suspects.

Among the material unearthed was a startling nugget: an Al-Qaeda figure, well known to MI5, had apparently slipped into the country unnoticed in mid-June, just three weeks before the bombings.

As MI5 agents delivered more information, their G branch handlers were told that the man, a 33-year-old Briton of Pakistani origins, had arrived at a seaport — possibly Dover, possibly Felixstowe — on a regular ferry. More alarmingly, a separate source reported that the man had flown out of the country the night before the attacks.

MI5 had a fat file on the suspect — call him M. He had extensive links to Al-Qaeda followers in America and was suspected of involvement in a previous Al-Qaeda attack. For the director of G branch, it was a deeply disturbing development and the alarm bells were ringing loudly.

Had M been the mastermind who met the London bombers and ordered them on their murderous mission? Had he told them which targets to hit? Why had he not been spotted arriving or leaving? Had MI5 missed the key man?

“There are no excuses,” said a senior security official. “Nearly 60 people are dead and it can’t be allowed to happen again. If it turned out that one of the bombers was someone we actually knew about then, I agree, that would be extremely difficult.”

Whether the mystery M was involved or not remains unclear (he may have been misidentified), but other awkward facts are emerging.

The early belief that the suicide bombers were “clean skins” completely new to the security services has proved wrong: at least one, possibly two, of the bombers were known to investigators in Britain and America. They had come across their names in operations a year ago.

In March 2004 police arrested eight main suspects after receiving intelligence about a potential attack. They found 600lb of ammonium nitrate fertiliser, a potential ingredient for a bomb, in a lock-up garage.

As they investigated the backgrounds of the men, the name of Mohammed Sidique Khan — who blew up a train at Edgware Road — surfaced. He was a peripheral figure to that inquiry and when MI5 made its assessment, it classified him as no threat.

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