The discovery of pin-pricks in vials of saline solution at Stepping Hill Hospital, Stockport, England, where three people have died, has triggered the hunt for a potential killer, police said.
Security has been stepped up at the hospital following the deaths of a 44-year-old woman with multiple sclerosis and two men with cancer aged 71 and 84. All three are thought to have received saline deliberately contaminated with insulin, along with 11 other patients who were affected but not seriously harmed.
A senior nurse who noticed an unusual number of patients on the ward with low blood sugar examined the vials and found evidence of tampering. "She went to the box and found the pin-pricks," a police spokesman said.
Analysis of the contents showed the saline vials had been injected with insulin, which is potentially lethal when given to non-diabetics.
The hospital contacted police on Tuesday; all vials of saline were checked, and only one batch was found to be affected. By then two of the patients had died. Later, following the checks, the 84-year-old man died.
Police said it was impossible to be sure what had caused the deaths until full post-mortem examinations had been carried out, and there might turn out to be no link with the contaminated saline. Two of the patients were reported to have been terminally ill, but the spokesman said that was in doubt. "It is a grey area whether they were terminal or not," he said.
Chris Burke, chief executive of Stockport NHS Foundation Trust, which runs the hospital, said all future deaths there that showed any sign of relevant symptoms would be referred to the coroner for investigation.
"It was the staff at the hospital who brought this matter to our attention and I've asked them to continue to be extra vigilant to help safeguard patients. We have increased security, and already replaced all saline ampoules across the hospital," he said.
Police said it was unclear what motive the perpetrator may have had.
Reports suggested it might have been a saboteur with a grudge against the hospital, or someone attempting to carry out a mercy killing.