Ginseng and John Purdey re Birmingham bombing
Just for the record, the basis of 90+% of all anti-terrorist work comes from informants.
For many obvious reasons, one cannot use those confidential informants as witnesses in court nor rely solely upon them – hence the need for forensic or other more tangible evidence (pre 911 anyway) to actually nail them.
That is to say, the police would have “known” who were probably involved but could not prove it.