"The number of people known to authorities is just too large for this to be feasible."
When the senior people in an anti-terrorism focussed muslim charity (who have experience and qualifications in such things) ring up and say "XYZ is acting bloody strangely, menacingly and in my opinion is dangerous" then attention should be paid to the quality of the source and weighted accordingly.
Because that's exactly what happened in this instance - and that wasn't the only source.
Ditto cases where multiple reports are being filed, especially from community leaders such as multiple Imans (which happened in Manchester)
It's one thing to "gather intelligence" and quite another to collate such reports and (not) act on them.