Mohammed Sidique Khan, one of the four suicide bombers who carried out last week's explosions in London [Images], was scrutinised by Britain's intelligence agency MI5 last year but let off after he was judged not to be a 'threat' to national security, a media report said today.

Khan, a 30-year-old teaching assistant from Dewsbury, West Yorkshire, who killed six other passengers when he blew himself up on an underground train at Edgware Road, was the subject of a routine threat assessment by MI5 officers after his name cropped up during an investigation in 2004, a senior government official said yesterday.

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But Khan was judged not to be a threat to national security and thus MI5 failed to put him under surveillance, the official was quoted as saying in The Sunday Times.

The inquiry focussed on an alleged plot to explode a 600-pound truck bomb outside a target in London, thought to be a crowded Soho nightclub. The intelligence agency had found out in 2004 that Khan had been visiting a house used by a man who had met one of the suspected truck-bomb plotters. However, MI5 officers subsequently decided that because Khan was only 'indirectly linked' to one of the bomb suspects, he was not considered a risk and the intelligence service took no further interest in him, the report said.

The government official said a 'quick assessment' had been made of Khan at the time and like hundreds of others linked to the inquiry, he was judged to be 'on the periphery' of the suspect cell's network.

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'You made quick assessments of them to decide whether or not they were a threat. None of the other people were a threat, including Khan,' he was quoted as saying in the newspaper.

The official conceded that MI5 might be accused of being at fault if it turned out that it had overlooked a terrorist suspect. 'MI5 is fair game at the moment. We've only got finite resources. You can only concentrate resources on those people who are a direct threat to national security.'