Police in Norfolk say an allegation has been made about the care a patient received at a Norwich hospital before he died.
A man in his late 40s, who had been receiving treatment at Hellesdon Hospital, had a medical emergency on Tuesday 1 October.
He was taken to the Norfolk and Norwich University Hospital, where he died three days later.
Detective Chief Inspector Tracey Little said the allegation related to "the care the patient received prior to and during the medical emergency" at Hellesdon Hospital.
"A Home Office post-mortem examination has been carried out but proved inconclusive and further tests will now be carried out," she added.
The death is being treated as unexplained.
Police said the investigation was not linked to "any other enquiries being undertaken by the Major Investigation Team".