Allitt, dubbed the "angel of death" was jailed for life in 1993 for killing four children with fatal doses of insulin and trying to kill nine other people.

Security has been stepped up at the hospital following the deaths, which are regarded as unexplained until the results of the post-mortem examinations.

A nurse at Stepping Hill Hospital raised the alarm this week after reporting abnormally high numbers of patients with low blood sugar levels.

Insulin was later found in a batch of 36 saline ampoules in a hospital storeroom. Detectives believe it was deliberately injected into the saline containers, which were used by at least two hospital wards.

They are now treating as suspicious the deaths of three people: Tracey Arden, a 44-year-old mother-of-two with multiple sclerosis who died on July 7, and two men, aged 71 and 84 and both with serious health problems, who died this week.

"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," a source at Greater Manchester Police said Saturday.

Assistant Chief Constable Terry Sweeney said Friday that all deaths at the hospital that might be connected were being referred to the coroner.

"As things stand, we do not know for sure if the contaminated saline contributed to the deaths of these three people," Sweeney said.

"It is a possibility we are looking at alongside the coroner, but we must stress that it will be extremely difficult to establish a link.

"That 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."

Tracey Arden's brother, Gary, said Saturday that she had appeared to be responding well to treatment just hours before she died.

"During the afternoon my mother and father had been to visit her, then they left her looking like she was recovering well," he said.

He added: "They left and then a relatively short while afterwards they received a call from the hospital that Tracey had taken a turn for the worse and they think they should come on and see her.

"By the time my mother and father arrived she had just passed away."

She had been suffering from multiple sclerosis for the past 12 years and had been in care, but her visit to Stepping Hill Hospital was "routine", he said.

He said it came as a "complete shock" to hear about the police investigation this week, which has forced them to delay her funeral.

Chris Burke, the chief executive of Stepping Hill Hospital, said Friday that staff there were working closely with the police investigation and had also been asked to be "extra vigilant to help safeguard patients".

"We have increased security both in terms of access to the hospital and access to medicines and already replaced all saline ampoules across the hospital," he added.