At least six people, who were deliberately poisoned with insulin between June and July 2011, have been confirmed dead, according to police.
Greater Manchester Police said that the deaths occurred at Stockport's Stepping Hill Hospital. The latest victims identified by the police are John Beeley, 73, and Linda McDonagh, 60. Beeley died on January 20.
Linda McDonagh was poisoned on January 14 while battling motor neuron disease and pneumonia at the hospital, her family said.
Her husband Steward was quoted by the BBC as saying that he suspected someone out there targeting vulnerable people.
Tracey Arden, 44, Arnold Lancaster, 71, and Derek Weaver, 82, died in July 2011. A fourth man, William Dickson, died on December 31, 2011. Around 21 people were thought to have been unlawfully administered insulin between June and July 2011.
"It is really important that we differentiate between the deaths of Tracey Arden, Arnold Lancaster and Derek Weaver - who died shortly after being poisoned - and the three subsequent deaths," said Simon Barraclough, the investigating officer.
He added: "We believe insulin may have played more than a minimal part in the deaths of Tracy, Arnold and Derek.
"While we are continuing to undertake stringent tests into the deaths of all six patients, we believe that it is highly likely the later deaths should more properly be attributed to natural disease. Due to the complexity of the medical conditions presented by all those who were poisoned and have since died, we have engaged the help of a team of highly experienced experts who continue to carry out comprehensive testing."
All the suspicious deaths took place in wards A1, A2 or A3. These are acute care wards meant for seriously ill patients.
Victorino Chua, a 46-year-old male nurse who is suspected of murdering three Stepping Hill patients, was released on bail in early January 2011, pending further inquiries.
Nurse Rebecca Leighton, 28, was released and cleared of all charges in September 2011 on the lack of sufficient evidence against her. She spent six weeks in jail in the course of the investigation.