Two elderly people have died at a hospital in north-west England because saline bags were contaminated with insulin.
Police said the patients, both in their eighties, died over the past week at Stepping Hill hospital in Stockport, near Manchester.
An inquest has already been opened into the suspicious deaths of three other patients, who had unusually low blood sugar levels.
Police believe the saline was deliberately sabotaged and are examining the treatment of 14 people.
On Wednesday, detectives arrested 27-year-old nurse Rebecca Leighton on suspicion of murder.
Police removed six bags of items and a computer from her flat.
They have not charged Leighton with any offence. and have a warrant to question her until Friday night.