.... Mentioned by one of our resident hand-wringers, right? "OOhhhh, the west is done, done, done!!! 
" They're as bad as the vigil-goers. All bluster and no rational thought.
I expect the British authorities are probably a lot more brutal about this stuff than people recognize, and for sure the French are. Granting the reflexive and well-nigh suicidal protectiveness of the left, they don't represent the feelings of most.
"Knowing about" is shaky grounds, though. They're keeping their eyes on probably hundreds or thousands of potential terrorists. They simply can't "do something" about all of them, and even if they're doing their best, the ugly truth is that every once in a while one will get past the guards. The problem is, in a country like the UK, governed by rule of law, subject to Constitutional constraints ... how do you keep it balanced?
Had he done anything for which he could actually be arrested? Consider carefully how you'd answer. Do you think you'd like to live in a place where being arrested "on suspicion" was the norm. After all, you post here -- to many on the left you're as suspicious and dangerous as the bomber was.
With any cell phone or other communications traffic indicating intent, "Possession of bomb making materials" has been used as a charge. Those suspected could be more closely monitored, but that takes people, budget, and the will to do so, within the rules. At some point the system becomes so severely infiltrated that the normal mechanisms of secrecy no longer remain secret, or even are not applied. (DOJ under Obama, for instance)
Unfortunately, the direction all this takes us in is one of Bills of Attainder and the tracking of every expenditure and purchase, all totalitarianism wrapped in the mantle of providing "safety", and equally to be rejected in a free society.
The only other way to exert control is to do away with the nonsense of "multiculturalism" when dealing with fundamentally incompatible cultures and retain separation. Too late to impose that without a pogrom.
