Four West Midlands police constables have been arrested following the death of a man who was detained by the force.
Kingsley Burrell, 29, was held under the Mental Health Act on 27 March 2011 and died four days later.
The Independent Police Complaints Commission [IPCC] said the four were held on suspicion of gross negligence manslaughter and misconduct in a public office.
West Midlands Police said it had co-operated fully with all IPCC requests.
The four officers placed Mr Burrell while he was under restraint into a seclusion room at a mental health unit in Birmingham, the complaints body said.
It said the four PCs were arrested after refusing to be interviewed by the IPCC.
The complaints body said the officers had been due to be interviewed last month but, after initially attending the agreed location, left the premises before any interviews could take place.
They have previously been interviewed by the IPCC under criminal caution.
A further 10 police officers have been interviewed as witnesses during the course of the investigation.
At the request of the IPCC, an investigation into the involvement of non-police personnel is being carried out by Dorset Police.
A spokesman for the complaints body said the independent investigation into Mr Burrell's death was well advanced and expected to conclude shortly.
West Midlands Police said on 27 March 2011 officers attended an incident in the Winson Green area of Birmingham and detained Mr Burrell under the Mental Health Act.
The force said that three days later officers were called to support medical staff at a mental health unit in the same area following reports of a disturbance.
From there Mr Burrell was transferred to hospital but later died, the force said.
In a statement, Assistant Chief Constable Garry Forsyth said: "As always, West Midlands Police has co-operated fully with all requests made by the IPCC during this investigation into the circumstances surrounding Kingsley Burrell's death.
"Police officers, when under investigation, have the same rights as any member of the public and can choose to submit written statements.
"We do not underestimate the impact this investigation has had on Kingsley's family and wider community, as well as those involved in the investigation.
"We share the family and community's concern that this investigation has taken such a length of time."