The police are now saying they think the bombers were killed at each of the four sites. I see that as encouraging news, because, most simply, they won't do it again. I do believe that terrorists ready to blow themselves up and able to merge into British society are very rare. Suicide bombing can be a regular tactic in Israel or Iraq, but not in Luton or Leeds.
Also, suicide bombers can be identified from their bodies, and the operation traced back from them, as seems to be happening currently. If the bombers are long gone, police have a more difficult job.
The police have moved quite quickly - it emerged in September 2001 that some of the US hijackers were already under investigation, but nothing concrete had come up, so they were being left alone. As soon as they struck, the authorities were able to quickly track down their backgrounds. The same may well be the true here, and no blame would neccessarily attach to the police or security services - It could be very difficult to make the jump from vague suspicion to grounds for arrest.
Why suicide bombing? Maybe they are too unsophisticated to produce timer devices of the neccessary reliability (not trivial - the "1996" bus picture in my post below was caused by an unintentional IRA suicide bomber). Maybe it is too difficult to leave a bag on a London train or bus -- we really are attentive to them after all these years. Maybe they feel that, other things being equal, it is better to die in the attack than survive it. I don't 100% believe the "Blood Feud" theory of Islamist terrorism -- I do think there is some strategy to it -- but it is valid to say that the bombers are very much concerned with themselves and their supporters, not just with their effects on us.
I'll liveblog the Met. press conference here if there's anything interesting. I'm not going anywhere - they (the Police, that is) have blown up another car in Luton station car park.
(times in GMT for reasons of insanity)
16:10 GMT - Peter Clark: They thought they knew who the bombers were before identifying their bodies.
16:11 GMT - Peter Clark: One of the suspected bombers was reported missing by his family around 10am Thursday.