Security has been stepped up at a hospital where three patients have died following the deliberate contamination of saline solution.
The deaths of a 44-year-old woman and two men aged 71 and 84 at Stepping Hill Hospital in Stockport remain unexplained while a further 11 patients have been affected but not seriously harmed.
However, Greater Manchester Police said it was clear that someone had been tampering with vials of saline and needed to be caught.
Describing it as a "fast-moving investigation", Assistant Chief Constable Terry Sweeney said: "We have someone deliberately contaminating saline in the one place that people should feel they are being most cared for.
"I want to reassure everyone connected to the hospital - staff, patients, visitors and the wider community - that we are determined to prevent further harm and to bring the offender to justice. We are working very closely with the hospital to improve the security of the patients and staff at the hospital."
As well as increased security patrols, he said he would also suggest to health trust bosses that anyone entering the hospital should be searched beforehand.
Mr Sweeney said it would be speculation to say the offender was a member of staff but he said all medical professionals at the hospital as well as patients would be spoken to in the next 24 hours.
The hospital contacted police on Tuesday after an experienced nurse reported a higher than normal number of patients on her ward with "unexplained" low blood sugar levels.
It concluded that some saline ampoules had been "interfered with" - with the contamination thought to have involved the use of insulin.
All future deaths at the hospital that show any signs of relevant symptoms will be passed to South Manchester Coroner John Pollard for the foreseeable future and may be referred to detectives to investigate.