The family of an 84-year-old man at the centre of the hospital medication tampering probe say they fear more patient deaths will be investigated.
Lung cancer sufferer George Keep died at Stepping Hill Hospital two days after a ward nurse raised concerns about a higher than normal number of patients with unexplained low blood sugar levels.
The suspicious deaths of Mr Keep, 71-year-old Arnold Lancaster and 44-year-old mother-of-two Tracey Arden are being linked to the deliberate contamination of saline solution.
Insulin was found in a batch of 36 saline ampoules in a storeroom close to ward A1 of the hospital in Stockport, Greater Manchester.
Detectives believe the insulin was deliberately injected into the saline containers which were used in drips by at least two wards, but they say the three deaths remain unexplained as they await post-mortem examination results.
The pensioner from Cheadle was admitted to Stepping Hill on June 27 after he suffered a broken hip in a fall. He was operated on and was recovering well until two days before his death when his blood pressure and blood sugar levels started to drop, said his daughter Carolyn Knowles.
She said her family were now "concerned" that the number of deaths that police are investigating will rise.
"If you go into hospital, it is supposed to be a safe place," she said. "It is a place where you trust people. You just don't possibly imagine anything like this could possibly happen. I just cannot imagine anyone wanting to do that to someone else. It is devastating. I just hope that it is not going to happen to anybody else and other families."
Long-term multiple sclerosis sufferer Ms Arden died on July 7, while Mr Lancaster died on Monday.
Police are continuing to interview doctors, nurses, porters, patients and visitors, while security remains high in and around the hospital. Eleven people who survived the effects of the tampered medication will also be interviewed.