From the link:
"...The mere presence of a drug, or its metabolites, in post-mortem tissue can be sufficient to reinforce suspicions of the link between the drug and the death. The conviction in 2000 of the English General Practitioner Dr Harold Shipman for the murder of 15 of his patients rested in part on the sudden demise of a group of otherwise healthy patients, for the most part elderly women, and in part on
the detection of morphine in skeletal muscle from the exhumed bodies of a subset of them, in the absence of evidence that they had been prescribed morphine or been in the habit of taking opiates [
12]. ..."
One victim, Kathleen Grundy, died June 24, 1998. Her body was interred just 5 weeks before being exhumed in August. Diamorphine was found in her body. "... a post-mortem revealed that she had died of a morphine overdose, administered within three hours of her death, precisely within the timeframe of Shipman's visit to her. ... " from
Dr. Harold Shipman | Murderpedia, the encyclopedia of murderers