The Independent Police Complaints Commission (IPCC) is investigating the circumstances surrounding the death of a 19-year-old man in Chandlers Ford.
It follows an incident when ambulance crews asked Hampshire police officers to attend an address in Heathfield Road at its junction with Nichol Road.
They were assisted with a young man who was behaving violently and had become unwell.
The man was taken by ambulance to the Royal Hampshire County Hospital in Winchester where he died later the same day.
Investigators are appealing for witnesses in Chandlers Ford, between 6.30am and 7.30am on June 27, to come forward.
The investigation is looking at the circumstances surrounding police involvement with the deceased, including restraint techniques used, from the point officers arrived at the address.