A further two patients have died at Stepping Hill Hospital after they were treated with contaminated saline solution.
Police say that the deaths of an 84-year-old woman, who died on 14 July, and an 83-year-old man who died on Thursday are now being investigated as suspicious. In addition a 41-year-old man remains critically ill. All were treated on the A1 or A3 wards which are the focus of the investigation, police said.
Tracey Arden, 44, George Keep, 84 and Arnold Lancaster, 71, all died at the hospital from what police believe was the deliberate contamination of saline solution with insulin.
Rebecca Leighton, 27, who is a nurse at the hospital, was arrested on suspicion of murder at her home address in Stockport, Greater Manchester, early on Wednesday morning.
Crime scene investigators were at the scene of the suspect's flat in Buxton Road, Heaviley - situated above a shop about a mile from the hospital where she works - throughout Wednesday. They left the scene at about 17:00, carrying up to six bags of items and a computer.
Neighbours said they thought Ms Leighton lived at the property alone but is often seen with a boyfriend or heading to work in her blue scrubs.
A man who works at a neighbouring shop said he knew Ms Leighton in passing, and added: "I wasn't here when she was arrested this morning but a load of police returned at 12.45pm and they have been in there ever since."
The Nursing and Midwifery Council (NMC) confirmed that Leighton had been arrested.
NMC chief executive Professor Dickon Weir-Hugh said: "The NMC has now received confirmation from the Greater Manchester Police Force Intelligence Branch and Stockport NHS Foundation Trust that registered nurse Rebecca Leighton has been arrested and is being questioned by the police.
"We have commenced fitness to practise proceedings with a view to suspending her from the register as quickly as possible. The NMC is the only organisation with the authority to protect the public by suspending a nurse's registration while they are under investigation."