A decision on whether the case will be referred to the Crown Prosecution Service will be made at a later date.
Five police officers are being investigated for alleged manslaughter after a 24-year-old man died following his arrest in Milton Keynes.
One of the Thames Valley officers is also being investigated for alleged common assault.
The inquiries by police watchdog, the Independent Office for Police Conduct (IOPC), come after the death of Brian Ringrose, who was taken to Milton Keynes University Hospital in an ambulance following his arrest on 27 January.
According to an IOPC report, the watchdog found evidence suggesting he was restrained by officers after he was medically discharged from the hospital.
They then took Mr Ringrose to a police van so he could be transported to police custody.
However, while on the way to the van, the report says "concerns were raised regarding Mr Ringrose's health and he was returned to the hospital where he was placed in an induced coma".
He died in hospital on 2 February.
While a post-mortem was carried out, the cause of death was inconclusive.
The IOPC investigation includes the use of a piece of equipment known as a "Flexible Lift and Carry System (FLACS)".
This was apparently used by officers to help them carry Mr Ringrose to the police van after he already had handcuffs and limb restraints put on.
The use of FLACSs has been temporarily suspended by Thames Valley Police.
According to the IOPC report, the five police officers, who have not been named, have been told they are under investigation for alleged gross negligence manslaughter and unlawful act manslaughter.
All of them have been served with gross misconduct notices for potential breaches of professional behaviour related to use of force and duties and responsibilities.
IOPC regional director for the South East, Graham Beesley, said: "Our thoughts are with Mr Ringrose's family and all of those affected by his tragic death.
"This is a significant development in our investigation and is based on evidence we have gathered to date.
"We have updated Mr Ringrose's family and will now seek to interview the officers under criminal caution. It is our role to independently investigate all of the circumstances when someone dies in police custody."
A decision on whether to report the case to the Crown Prosecution Service will be made at the end of the IOPC investigation.
Earlier this month when the investigation was announced, a spokesperson for Thames Valley Police said: "This incident is being investigated independently, as such it would not be appropriate for us to offer further comment at this time."
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