There will inevitably be some people close to those who have committed the atrocities who might have suspected they are becoming radicalised - friends and family.
However making the jump from thinking someone is acting strangely, to reporting them to the police is a very big one.
You might see a parent acting strangely around a child, or a parter in a relationship looking subdued, but it takes a lot to actually call social services and report suspected abuse, particularly if it is about someone you know.
If there was a financial reward for anyone reporting potential terrorrist crimes in respect of gaining a conviction, would more be prepared to grass on their mates? If a pal was up to no good, how's about a new pair of trainers or a decent phone for info??
The mere fact that they couldn't trust their "mates" might break up a cell-to-be, even if no-one actually did. Wasn't that how the IRA eventually came to heel, they couldn't trust anyone, least of all themselves?
Sounds very much like East Germany who relied on informants and secret police, but the situation has got to the point where "why not?"