A nurse has been arrested on suspicion of murder after the deaths of three patients at a hospital in Stockport, Greater Manchester.
Rebecca Leighton, 27, of Heaviley, Stockport, was arrested at home and is being questioned after two men and a woman died at Stepping Hill Hospital.
Police believe insulin was deliberately injected into saline containers.
Sixty detectives have been involved in the inquiry and officers have so far questioned more than 50 people.
Police have been stationed outside Ms Leighton's flat on Buxton Road, where earlier officers removed six bags of items and a computer.
The Nursing and Midwifery Council said proceedings were under way to suspend her nursing registration while she is under investigation.
On her Facebook page Ms Leighton describes herself as engaged, and calls herself a "happy go lucky kinda gal".
She states that she is a staff nurse, adding: "god it hard work x". She adds that she "loves the weekend (if im not workin) and having a laugh with the people that I call friends for a reason x".
Tracey Arden, 44, 71-year-old Arnold Lancaster, and George Keep, 84, died at the hospital in the past month.
A fourth patient, a man in his 40s, remains critically ill.
Greater Manchester Police believe the sabotaged saline containers were used on at least two wards, A1 and A3.
The force said they were working on the assumption the contamination had taken place on site.
The four patients are among a total of 14 whose treatment since 7 July is being examined by police.
Dr Chris Burke, chief executive of Stockport NHS Foundation Trust, said: "We are aware that Greater Manchester Police have made an arrest this morning of a 27-year-old female.
"As the police are continuing with their enquiries, we are unable to provide any further details, but it is important to stress that no charges have been made at this stage."
A hospital spokesperson said it hoped to return to full business by 0900 BST on Thursday, although security measures would remain in place.
Detectives were called to the hospital after an experienced nurse reported a higher-than-normal number of patients on her ward with "unexplained" low blood sugar levels.
Officers found insulin had been used to contaminate a batch of 36 saline ampoules in a storeroom close to ward A1.
Security has been increased on the hospital site and staff have been told to work in pairs when they check or administer drugs.
Senior NHS bosses at the hospital are having daily meetings with police.
Insulin is always stored in a fridge in a locked treatment room but saline has also been locked away as a result of the incidents.
South Manchester coroner John Pollard has opened and adjourned inquests into Ms Arden, Mr Lancaster and Mr Keep.
Police revealed a damaged bag containing saline solution was found in the maternity ward but it had not been contaminated.
Assistant Chief Constable Ian Hopkins, leading the inquiry for Greater Manchester Police, said: "It is important to stress that this product has not been contaminated and, in fact, the damage could very well be accidental, as can happen quite easily in a busy hospital environment.
"Extra security measures are in place across the hospital and staff continue to be vigilant, which of course, is to be expected.
"Due to heightened awareness, staff are alerting police to anything they feel might be suspicious or relevant to the investigation and to date there has been no further contamination of any product since the control measures were introduced."