Police were today given more time to question a 46-year-old male nurse suspected of murdering three hospital patients.
Victorino Chua, 46, was arrested on Thursday on suspicion of tampering with medical records at Stepping Hill Hospital in Stockport, Cheshire.
Detectives are probing the poisoning of patients at the hospital last summer.
Last night Greater Manchester Police said the nurse was now being questioned on suspicion of three counts of murder and 18 counts of causing grievous bodily harm.
Today they were given another 24 hours to question Chua.
Detectives have obtained a further warrant of detention that is due to expire at 10.10am tomorrow.
Assistant Chief Constable Terry Sweeney said last night: "Firstly, and most importantly, I must stress that we have not established the degree to which deliberate contamination of products may have contributed to the death of four patients.
"We have taken the decision to arrest this man on suspicion of murder in close consultation with the CPS and it is important that no-one jumps to any conclusions because of these steps or second guesses where our investigations will take us.
"This specific inquiry, and the investigation as a whole, is a difficult and complex piece of work that requires very detailed forensic and medical analysis, which is ongoing.
"We know that the victims and their families want answers. We are acutely aware of this and are working very closely with them as well as hospital staff and the local community to minimise the upset caused and to keep them fully updated of any and all developments."
The suspect is being questioned over the contamination of saline drips in June and July last year, causing patients' blood sugar to fall in "hypoglycaemic episodes".
All the deaths took place on wards A1, A2 or A3 - acute care wards caring for seriously ill patients.
Officers investigating the deaths are also looking into a fourth death.
Police are looking into whether the alleged poisoning of William Dickson, 82, suffered at the hospital during the summer was a factor in his death.
The former journalist, of Cheadle Hulme, Cheshire, died on New Year's Eve.
Police said 21 patients were believed to have been deliberately poisoned. The cases of a further 21 patients who suffered hypoglycaemic episodes are not being treated as suspicious.
Chua is one of 650 people already spoken to by police during the complex and long-running investigation at the hospital since the saline drip contamination was discovered.