All that surveillance and he still managed to do what he did.
what sort of surveillance do you want? He'd done nothing to warrant being locked up; when he grabbed a knife in a shop, he was dead within moments. Much better than announcing after a mass murder that the suspect was "known to the authorities"; there, the authorities were almost on top of him, and he was the only person to die. My sympathies to those hurt, but it was close to being a perfect counter-terror action.