ON JULY 7, 2005, in London, shortly before 9 a.m., three suicide bombers blew themselves up and destroyed the subway cars they were riding in, killing 39 and injuring nearly 700 commuters. About an hour later, another suicide bomber got on a double-decker bus--crowded with men, women, and children who jumped on board following the closure of the London Underground--and detonated the bomb hidden in his rucksack; this attack killed 13 more people and injured more than a hundred. Although aware of a high threat posed by Islamic radicals at the time, the British government was nevertheless taken completely by surprise by the most deadly attacks ever in peacetime Britain.

Now two long-anticipated government reports on the bombings have been completed and made public. The first, the Blair government's "official" version of the 7/7 attacks, is a narrative of what happened and an overview of what was known and not known at the time about the bombers. Its concluding note is that the "case demonstrates the real difficulty for law enforcement agencies and local communities in identifying potential terrorists."

The second report was issued by the Intelligence and Security Committee, the Parliament's cross-party oversight body for Britain's domestic and foreign intelligence community. Somewhat more analytic and investigative, the committee's report attempts to assess how well British intelligence, especially MI-5, the domestic security service, performed in the run-up to the bombings, whether any intelligence was overlooked that could have prevented the attacks, and what lessons can be learned from what did happen. Its conclusion: With hindsight, some leads were probably missed that could have increased the chances of preventing the attacks, but--given the number of urgent, higher priority investigations going on and the reality of limited MI-5 resources--the decisions not to follow up on those leads "were understandable." Echoing the limitations theme of the government's report, the committee concludes that it has "been struck by the sheer scale of the [terrorist] problem that our intelligence and security agencies face and their comparatively small capacity to cover it."

And, to the extent the two reports are accurate, the difficulties British intelligence and security services faced in uncovering the terrorist plot were indeed daunting.

To start, none of the four suicide bombers stood out as an obvious Islamic extremist. Mohammad Sidique Khan, the apparent leader of the group, was ethnically Pakistani but a second-generation British citizen, as were two of the other bombers. (The fourth bomber was Jamaican born, had moved with his mother to Britain as a youngster, and converted to Islam after his mother did in 2000.) While religious, Khan didn't openly avow radical, violent behavior. To the contrary, he had spoken out against the al Qaeda attacks of 9/11. This was not true of two others in the group but, again, they were no different from thousands of other British Muslims and, more important, were not known associates of any would-be terrorists that MI-5 or the local Special Branches would have been tracking. Nor were any of the four embedded in some deeply impoverished, alienated subgroup of immigrants. All in all, Khan and the other suicide bombers seemed hardly out of the ordinary when set against a sea of other Muslim immigrants who had poured into Britain over the previous decades.