Thames Valley Police's contact with the 24-year-old man before his death is currently being investigated.
The Independent Office for Police Conduct (IOPC) says that they have allegedly established that the man, from Milton Keynes, was detained by officers on Wednesday 27th January 2020 following reports of a disturbance.
He was then taken to the hospital by ambulance after medical concerns were raised to police.
A spokesperson for IOPC said: "Evidence we gathered indicates that later that day as the man was being discharged into police custody there was an incident that led to him being restrained by officers. After being taken to a police vehicle he suffered a cardiac arrest and was returned to the hospital where he was placed in an induced coma."
An independent investigation was declared on 28th January, and the man passed away at Milton Keynes Hospital on 2nd February.
The IOPC states that HM Coroner has been informed and an inquest is scheduled to take place in due course.
Graham Beesley, IOPC Regional Director, said: “I would like to send my condolences to the man’s family and everyone else affected by his death at this difficult and sad time.
“We have a duty to investigate in these circumstances and in particular we are looking at the contact that police officers had with the man prior his death.
“We have contacted the family to explain our role and we will keep them regularly updated throughout our enquiries.”
The IOPC adds that investigators have gathered police body-worn video footage for review, alongside hospital CCTV footage. Initial accounts have also been obtained from officers.
A spokesperson for Thames Valley Police said: “Our thoughts remain with the man’s family at this very difficult time, who are being supported by the IOPC.
“Thames Valley Police has been fully co-operating with the investigation from the outset and will continue to do so.
“This incident is being investigated independently, as such it would not be appropriate for us to offer further comment at this time.”
