Conversation so far:
What was the detection of Harold Shipman?
Harold Shipman was detected when concerns were raised about the high death rate among Shipman's patients.
Are there any other interesting aspects about this article, besides the detection of Harold Shipman?
In particular, she was concerned about the large number of cremation forms for elderly women that he had needed countersigned
What was Harold Shipman being investigated for?
Harold Shipman was being investigated for murder
How did Harold Shipman kill them?
Shipman killed his victims by administering lethal doses of diamorphine (heroin), signing patients' death certificates, and then falsifying medical records to indicate that they had been in poor health
When was Harold Shipman caught?
Shipman was arrested on 7 September 1998

Question: How did the police catch him?
Self-contained rewrite: How did the police catch Harold Shipman?