The trouble is you may not know they pose a threat until the attack is happening. Saturday's attack was carried out with a hire van and a couple of knives: Nothing that is likely to immediately flag up as danger. You can't interrogate every Muslim bloke who has been to Syria in the last 5 years simply because they rent a Transit.
Ultimately, it's going to be the use of undercover ops, some clever bids at GCHQ and most of all the Muslim community that's going to prevent further attacks.