Shahzad Tanweer, 22, who detonated a rucksack bomb on the Tube train at Aldgate, is believed to have been indirectly linked to an alleged plot to build a bomb in 2004. It has already been established that the suspected mastermind, Mohammad Sidique Khan, 30, had been known to security services.
The disclosure that a second of the four bombers had come to the attention of MI5 is likely to increase pressure for a public inquiry into the London attacks and any failures in intelligence. Charles Clarke, the Home Secretary, is resisting setting up a wide-ranging independent inquiry, instead opting for a more limited "narrative" led by a civil servant. The Independent has also established that there are so many new terrorist suspects coming to the attention of the security agencies and Scotland Yard's anti-terrorist branch that there are not enough officers to investigate them all.
Tanweer, who killed eight people including himself, is believed to have been the subject of a routine assessment by MI5. No further action was taken after it was decided that he was on the periphery and that there were far more significant suspects to investigate. The decision by MI5 to disregard two of the men who would later become suicide bombers was based on the assessment that they were not on the intelligence "radar" and only had an indirect link - via an associate of the gang under investigation - to the main targets.
But the disclosure highlights the police and security services' lack of intelligence on a growing number of British-born Muslims who have become radicalised while in the UK, with others learning terrorist tactics in Pakistan, Afghanistan, and north Africa.
In the immediate aftermath of the July 7 suicide bombings it was thought that the terrorists, Tanweer, Khan, Jermaine Lindsay, 19, who killed 26 people at King's Cross, and Hasib Hussain, 18, who killed 13 people on the No 30 bus at Tavistock Square, were "clean skins" with no known links to terrorism.
But it later emerged that Khan, a teaching assistant from Dewsbury, West Yorkshire, who killed six other passengers when he blew himself up on a Tube at Edgware Road, was the subject of a routine threat assessment by MI5 officers after his name cropped up during an investigation in 2004.
Similarly, until now Tanweer, from Beeston, Leeds, was not thought to have been looked at by the security services. But as anti-terrorist officers have pieced together the background of the suicide bombers, more evidence has emerged of their previous involvement and contacts with extremists.
Tanweer and Khan spent three months in Pakistan before returning to Britain in February this year. It is thought they could have been given terrorist training in religious schools in Pakistan. Lindsay was also briefly on a terrorist watchlist, according to US officials, although that report has been dismissed by British security sources as a case of mistaken identity.
In a separate development, authoritative sources have confirmed that some terror suspects, who officers would like to put under surveillance, are not being scrutinised because of a lack of resources. Despite big increases to the resources being made available to counter terrorism there are still not enough people to investigate the growing pool of terror suspects.
Intelligence on an unprecedented number of British-based al-Qa'ida suspects is being uncovered. "We do not have enough people to look at everyone. We are having to prioritise cases," said a counter-terrorist source.
A different source added: "Since the 9/11 attacks in the United States there has been an explosion in numbers of would-be terrorists, both here and abroad."
Security and police officers have foiled three alleged terrorist plots in the past five months, but there is a growing belief within MI6, MI5, and Scotland Yard that a "successful" attack in Britain is inevitable in the coming year.
MI5 has just boosted its numbers from 2,000 to 2,500, and is looking to hire at least a further 500, while the Met has asked for funding for an extra 1,500 anti-terrorist officers.
Counter-terrorist sources have suggested that in future it could be that success will be judged not on whether all the attacks are foiled, but how few get through.
Sir Ian Blair, the head of the Metropolitan Police, has repeatedly warned that more terror attacks are being planned.
He recently said: "The sky is dark. Intelligence exists to suggest that other groups will attempt to attack Britain in the coming months." On the issue of whether a second bomber had been previously investigated, a Scotland Yard spokesman said: "We are not prepared to discuss [this]."
The unanswered questions
By Nigel Morris
* THE POLICE RESPONSE
Why was the terror alert downgraded before the beginning of July? The July 7 attacks, and the failed bombings a fortnight later, took MI5 and the security agencies completely by surprise. The level of the security alertin London was lowered in June as confidence grew that terror networks did not have the ability to organise a major attack. Was Sir Ian Blair, the Metropolitan Police commissioner, guilty of complacency by insisting before the explosions that the capital's security was "the envy of the policing world"?
* THE BOMBERS
Why did the security services lose track of Mohammad Sidique Khan? Khan, who set off the Edgware Road bomb and is believed to have masterminded the attacks, had been investigated by MI5 after he was recorded speaking to a terrorist involved in a failed bomb plot last year.
He was cleared after investigators concluded he had only the most peripheral involvement with al-Qa'ida associates.
Was a vital piece of intelligence about Khan, who is believed to have made contact with terrorists overseas, overlooked? Was there a fifth bomber? More explosives were found inside a rucksack in the car left by the bombers at Luton station, suggesting a fifth man could have been involved. Have police resolved this question? And if there is a fifth man, is he still at large?
* THE SECOND ATTACK
Were the July 7 bombers linked to the alleged July 21 attackers? And could a third cell still be at large?
Circumstantial connections have been established between them, leading Charles Clarke, the Home Secretary, to conclude that it would be "very surprising" if they were not linked. If there was a connection, does it mean there is a wider loose network of terrorists lying low plotting their next outrage?
* THE MASTERMIND
Who was ultimately behind the attacks? There has been persistent speculation that an unidentified Pakistani man - seen with the suicide bombers on a whitewater rafting trip as well as at a Yorkshire community centre frequented by Khan - oversaw the operation. What do the security services know about him and how firm do they believe were the contacts between the bombers and senior figures in the al-Qa'ida network?
* THE EMERGENCY RESPONSE
Were mistakes made in the handling of the disaster? The emergency services have been widely praised for their speedy response to the succession of blasts of July 7, but questions remain. Why were the explosions on the Tube put down to electrical failures minutes after the real explanation should have been apparent? Given that the final blast took place on a double-decker bus in Tavistock Square, was it a mistake to keep buses running after the Tube blasts?
* THE MOTIVATION
Were the attacks motivated by the Iraq war? This is the question ministers are desperate to avoid. They deny any link can be made, but Muslim leaders insist it cannot be dismissed as a factor. Khan claimed in a video message that the attacks were in response to "atrocities" committed by the West against Muslims.
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