A report into whether the security services could have done more to prevent the London 2005 suicide bomb attacks is due to be published today.
The probe into the July 7 bombings which killed 52 commuters in the capital's worst peacetime attack was ordered after it emerged that two of the four young British Islamists who carried out the bombings had been known to the security services.
In the aftermath of the blasts on three underground trains and a bus, the government said that the bombers were "clean skins" who had not previously crossed the authorities' radar.
But evidence in recent trials of terrorism suspects has revealed that two of the men, Mohammad Sidique Khan and Shehzad Tanweer, had been watched, photographed and recorded by surveillance operatives a year before the attacks.
They were seen meeting Omar Khyam, the leader of an al Qaeda-inspired gang who were jailed in 2007 for plotting to bomb nightclubs, trains and shopping centres in Britain using fertiliser-based explosives.
The Intelligence and Security Committee (ISC) is now due to unveil the results of an investigation into what the police and the MI5 domestic security agency actually did know.
It is the second report by the ISC into the bombings. An initial examination in 2006 acknowledged for the first time that the security services had come across Khan and Tanweer.
But the parliamentary panel did not blame officials for not pursuing Khan and Tanweer, saying there was no indication in 2004 that they were significant.
Relatives of victims and survivors of the 7 July bombings have long argued that there should be a public inquiry.
They say there are still fundamental questions that need answering and that the ISC, with limited powers and a lack of accountability, would not enjoy public confidence.
One of their main queries centres on why neither MI5 nor the police kept an eye on Khan after the fertiliser plot suspects had been arrested.
"Personally, I'm not optimistic that this report will achieve anything close to the kind of in-depth investigation we need," said Jacqui Putnam, who was on board the train blown up by Khan, told Reuters.
"Up to now, all the documentation the government has produced on 7/7 has been shoddy and inaccurate," she told Reuters.
The government has ruled out an inquiry, saying it would sidetrack the security services when Britain is at serious risk of a terrorist attack. Britain remains at its second highest threat level, "Severe", meaning an attack is highly likely.
The security services have said pressures on resources and the fact that neither Khan nor Tanweer had been identified as priorities meant that the decisions taken at the time had been reasonable.
MI5 has even taken the highly unusual step of providing facts to dispel "rumours" about the bombings on its website.