Detectives are questioning a 27-year-old nurse over the deaths of three patients at a hospital.
Rebecca Leighton was arrested on suspicion of murder at her home address in Stockport, Greater Manchester, early on Wednesday morning.
The unexplained deaths at Stockport's Stepping Hill Hospital of Tracey Arden, 44, George Keep, 84 and Arnold Lancaster, 71, are all being linked to the deliberate contamination of saline solution with insulin.
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 5pm, 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 Ms 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."