British security officials confirmed Thursday that the suspects were known to MI5, the domestic security agency, in the years before the attack, which stunned many people with its sheer brutality.  http://www.nytimes.com/2013/05/25/world/europe/london-attack.html?pagewanted=1&src=twr    What British security officials knew about the two men held after the London attack remained unclear. But unidentified officials who spoke with British reporters said that both men had appeared on lists of people known to have been involved with Islamic militant groups that have been under surveillance by agencies, including MI5, that form the front line in Britain’s counterterrorist operations. News coverage of the killing in Britain had included “the point that the two suspects in this horrific attack were known to the security services,”
Security officials were quoted by the BBC as confirming that one of the suspects, Mr. Adebolajo, had come under surveillance in recent years when he attended meetings of Al Muhajiroun, a militant group that was later banned.British security experts noted that several of the militants involved in the 2005 transit bombings had been known to MI5 for years before the attacks, but that MI5 officers had reviewed their cases and decided that they were peripheral figures, apparently not involved in terrorist plotting.
The decision not to keep track of those men, the experts said, may prove to have been similar to the case of Mr. Adebolajo.