"Some of those returning will undoubtedly try to kill again. We have no idea who they will be!"

If they are returning Jihadhi's I have sufficient faith in our security forces that each and very one them will be very well known.

With 23000 Jihadis at home and others returning from foreign climes, yes I am sure they know all their names and what papers they read and what risk each one of them poses.
"As a recent study says:“National resources in most countries are insufficient to monitor more than a handful of returnees” (Barrett, 2014). And surveillance of such individuals may have to continue for months or years. Whether all returning fighters should be promptly arrested or dealt with more discriminately is a topic of debate in Europe."
https://www.thetimes.co.uk/article/huge-scale-of-terror-threat-revealed-uk-home-to-23-000-jihadists-3zvn58mhq

I suspect something a tad more tangible than faith is required to sort the problem. But ignorance is bliss(so they say)


https://www.rand.org/content/dam/rand/pubs/perspectives/PE100/PE130-1/RAND_PE130-1.pdf