Detectives investigating three hospital deaths are to interview 11 patients who survived the effects of tampered medication in a bid for clues to find the poisoner.
A 44-year-old woman, named locally as Tracey Elizabeth Arden, a 71-year-old man and a 84-year-old man died following the deliberate contamination of saline solution at Stepping Hill Hospital in Stockport, Greater Manchester.
Insulin was found in a batch of 36 saline ampoules in a hospital storeroom close to Ward A1 after a nurse reported a higher than normal number of patients on her ward with unexplained low blood sugar levels.
The alarm was raised on Tuesday but multiple sclerosis sufferer Ms Arden, from Stockport, died on July 7. The two male victims, who both had serious health problems, died this week.
Greater Manchester Police's Major Incident Team is currently focusing its investigation from July 7 onwards but will review previous deaths at the hospital if new information comes to light.
Meanwhile, increased security measures have been put in place at the hospital to protect patients currently receiving treatment there.
Detectives believe the insulin was deliberately injected into the saline containers which were used by at least two wards but they say the deaths remain unexplained as they await post-mortem results.
A police source said: "We are no longer treating the tampered medication as a sole contaminated batch because we cannot be sure that this was the only incident of its kind.
"We will be interviewing all those who became ill from the contaminated saline to find out if they can shed any light on who is responsible."
One of the 11 surviving patients became very ill from her saline drip and has since recovered but the others were not thought to be seriously affected.
Security patrols have been stepped up as police methodically work their way through the hospital in interviewing doctors, nurses, porters, patients and visitors.
Police have suggested to the hospital that they put measures in place to individually search everyone at the entrances.
Mark Hunter, MP for Cheadle, said: "This is obviously a very serious matter, I have to say I think the hospital have acted entirely responsibly, acted very promptly calling the police in as soon as they knew here was a problem. And the really important thing now is that the police are allowed to conclude their investigations as a matter of some urgency to provide reassurance to the wider community."
Mr Hunter said there had not been previous concerns about security at Stepping Hill Hospital.
He said: "I've certainly heard from many constituents the last few days regarding their concerns over what's happened here.
"But we shouldn't forget there are many dedicated professionals at work at Stepping Hill Hospital and this is also a hospital that's been held in almost universally high regard by the community it serves.
"Many of us around here have good cause to be grateful for the standards of care that we've enjoyed at Stepping Hill over the years.
"But that isn't to deny at all this is a most serious matter.
"One death is one too many. People go to hospital to try and get better. They don't expect to get worse whilst they're in hospital, and frankly we need to get to the bottom of this and we need to get to the bottom of it urgently."
Mr Hunter said he was "very reluctant" to draw conclusions at this early stage of inquiries as to whether the cause of the three deaths had been the result of tampering with saline solution.
He also said he had not spoken directly to relatives of the three deceased patients.