Michael Adebolajo and Michael Adebowale were "known" to MI5 before they killed Fusilier Lee Rigby in Woolwich but were considered to be of no immediate threat to anyone.
In the language of the intelligence agencies, that means the Security Service was aware of their Islamic extremist views and their association with others but had no evidence they were planning any attack.
Was that the right judgement, or should MI5 have kept a closer eye on them and, if it had, could it have prevented Fusilier Rigby's death?
Whitehall sources said both men had come to MI5's attention in various investigations over several years, two among several thousand known Islamic extremists.
One source said: "It is not surprising that someone who does something like this should have come to our notice before, given the range of our investigations."
The source suggested the agency does not believe it should have given the two killers a greater priority, based on what it knew about them at the time, nor does it argue it would have done more with greater resources.
In another favourite spook phrase, the pair were "on the radar, but not under the microscope".
MI5 accepts there will be lessons to learn from the way it dealt with them.
It is known that in Kenya in 2010, Adebolajo was arrested, appeared in court and deported over his plan to travel to neighbouring Somalia and join the terror group al Shabaab.
One British newspaper report said Adebolajo was able to return to Kenya last year, despite its government warning the UK he was a "dangerous radical". It was reported he may have used a false passport.
He is thought to have joined the UK-based extremist group al Muhajiroun in 2003, several years before it was banned and its leader thrown out of Britain.
Adebolajo appeared at the group's demonstrations and was arrested during scuffles outside the Old Bailey in 2006.
According to a close friend, MI5 was doing more than just monitoring Adebolajo. It asked him to work for the agency six months before the Woolwich killing.
Abu Nusaybah told BBC's Newsnight that Adebolajo was stopped and questioned on his return from a trip to Kenya, and later followed and approached at home by MI5.
Mr Nusaybah said: "He was basically being harassed. His wording was, 'they are bugging me, they won't leave me alone'.
"Initially, they wanted to ask him if he knew certain individuals. But after him saying that he didn't know these individuals, what he said was they asked him if he would be interested in working for them.
"He was explicit in that he refused to work for them, but he did confirm he didn't know the individuals."
Straight after the interview, Mr Nusaybah was arrested by counter-terror police as he left the TV studios. He is to stand trial next year on terrorism charges unrelated to Woolwich.
Adebolajo's brother Jeremiah has claimed the foreign intelligence agency MI6 tried to recruit him and urged him to help "turn" Michael to work for MI5.
It will be up to parliament's Intelligence and Security Committee to assess whether MI5's judgement of Adebolajo and Adebolawe was correct.
The committee of MPs is investigating the role of various agencies before the Woolwich attack and is expected to report early next year.
Its chairman Sir Malcolm Rifkind said soon after Drummer Rigby's death: "I don't think MI5 is in the dock. I think that would be very unfair."
MI5 has had a huge budget increase in the past decade, with the number of staff almost doubled, the opening of regional bases and the concentration of 90% of its resources on fighting international terrorism.
It was mildly criticised for poor record-keeping and police liaison by the ISC after the 7/7 London bombings of 2005, but escaped any censure.
The committee concluded that although the 7/7 ringleader Mohammad Sidique Khan was on its radar, the agency couldn't have been expected to identify him as a bomb plotter.
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