By Gordon Corera
BBC security correspondent
Over the past year, seven men stood trial at the Old Bailey accused of conspiring to build a massive homemade bomb out of fertiliser. The months of evidence revealed what MI5 really knew about the London suicide bombers. So why did the Security Service not act?
Identity: What did MI5 know about the bomber?
In March and April 2004 police involved in what became known as Operation Crevice arrested plotters involved in a conspiracy to build a massive fertiliser bomb in the UK.
Mohammad Sidique Khan, one of the four men who would go on to be London's suicide bombers, was not part of that conspiracy - but was linked to some of the plotters.
Amid the details to have emerged from the trial, MI5 is facing questions over whether it could have acted to stop Khan.
MI5 had classified him as a "desirable" rather than an "essential" target and he was therefore not investigated in the immediate wake of the March 2004 arrests.
But counter-terrorism officials also argue that, based on the intelligence they had, it was impossible to know who he was at the time - or what he would go on to do on 7 July the following year.
So what happened?
Khan was registered by MI5 as a contact of Omar Khyam, the ringleader of the fertiliser plot, and was spotted during the surveillance operation into Khyam.
MI5 AND CREVICE
Seven men charged - two found not guilty in April 07
55 individuals listed
15 judged essential targets
Khan and Tanweer listed desirable
No specific intelligence of terrorist intentions
Mid-2004 MI5 refocuses on urgent threats
Officers followed him to his West Yorkshire home - but he was never formally identified by name and never fully investigated. Critics say he could have been identified and following up on him might have prevented the attack the following year.
Whitehall officials concede this may look bad, but they argue it is only with hindsight it looks like a mistake.
They argue contact between Khan and the fertiliser plot conspirators was principally based around financial fraud rather than terrorist activity.
With finite resources, only certain individuals could be followed up, based on a system of prioritisation.
It is that system that was at heart of the decision making, say officials.
Following the Operation Crevice arrests, "essential targets" were followed up on - but "desirable targets" were given less priority.
Essential targets were those assessed likely to be directly involved in, or have knowledge, of plans for terrorist activity.
Desirable targets were those associated with terror suspects - but where there is no evidence of their direct involvement.
Only those contacts judged to be involved in planning attacks were placed on the priority list.
An avoidable mistake?
So was this a mistake and could the 7 July bombings have been avoided?
The Intelligence and Security Committee, charged with looking into the workings of MI5, said it was possible that the chances of preventing 7/7 might have been greater had different investigation decisions been taken.
MI5: Under scrutiny - and under pressure
But it also argued the decision not to give greater priority to both Khan and Shehzad Tanweer, his fellow bomber, was "understandable".
Senior counter-terrorism officials say that when Khan was on their radar in February 2004 there was no evidence he was actively planning the 7 July bombings.
In fact, they believe his planning only accelerated after he returned from Pakistan early in 2005: even if he had been investigated at the time it may not have led to any further action.
It is of course possible though that investigating him further - or asking the police in West Yorkshire to do so - might have led to warnings signs of the 7/7 plot.
But one of the critical issues at the time was MI5's workload, say officials.
Just three months after the arrests, another major operation began. This eventually led to the jailing for life of Dhiren Barot, a man who was planning a wide range of attacks.
This investigation sucked in resources to such an extend that officials say the Security Service had no slack to pursue "loose ends" like Mohammad Sidique Khan.
In a speech last November, the outgoing head of MI5 Dame Eliza Manningham-Buller provided a pre-emptive defence of her organisation against the criticism she knew was coming.
"I wish life were like (TV Drama) Spooks, where everything is (a) knowable, and (b) soluble by six people," she said. "We are faced by acute and very difficult choices of prioritisation.
"We cannot focus on everything so we have to decide on a daily basis with the police and others where to focus our energies, whom to follow, whose telephone lines need listening to, which seized media needs to go to the top of the analytic pile.
"We won't always make the right choices. And we recognise we shall have scarce sympathy if we are unable to prevent one of our targets committing an atrocity."
Officials warn that even with its rapid expansion, the scale of activity MI5 is witnessing - 200 networks in the UK, according to Dame Manningham-Buller - means that it is operating at full stretch and constantly having to make similar decisions to those it made in 2004.
"We're not the Stasi, we can't cover everyone," said one senior Whitehall official.