A church caretaker died after a webbing belt was unlawfully wrapped around his face while he was in police custody, a court heard.
Thomas Orchard, 32, died in hospital seven days after being arrested and brought to Heavitree Road police station in Exeter, Devon, in October 2012.
Bristol Crown Court heard Mr Orchard was held down, handcuffed and placed in restraints - with one allegedly used to carry his head.
He was then freed from the restraints - making "little or no movement" - and left lying face down on a blue mattress in his locked cell.
Officers did not re-enter the room for a further 12 minutes. By that time he had suffered a cardiac arrest and was not breathing, a jury heard.
Custody sergeant Jan Kingshott, 45, and civilian detention officers Simon Tansley, 39, and Michael Marsden, 56, deny Mr Orchard's manslaughter.
Their retrial heard Mr Orchard was arrested for a public order offence on Sidwell Street, Exeter, at about 11am on October 3. He died in hospital on October 10.
Mark Heywood QC, prosecuting, said: "He was physically restrained in some way or another, in the street and at the custody unit, for a total period of 22 minutes.
"For a significant part of the later period, that is at the custody unit, he also had an emergency response belt (ERB) applied over the whole or part of his face, including his nose and mouth at times.
"Once released from restraint in the cell, he made little or no movement. When the cell was entered 12 minutes later, he was in cardiac arrest."
The three defendants each deny one charge of manslaughter, alleging that they unlawfully killed Mr Orchard by gross negligence.
Mr Heywood said the combination of force, physical restraints and a "complete failure" to observe Mr Orchard closely led to him being starved of oxygen.
"No-one of those directly responsible took sufficient care to see that he was breathing properly - or at all," he told the jury.
"Instead, he was left in a locked cell, under remote observation for a further 12 minutes until his true condition was discovered. By then, it was too late."
A pathologist gave the cause of Mr Orchard's death as severe hypoxic-ischaemic brain damage, prolonged cardiorespiratory arrest following a violent struggle and a period of physical restraint including a prolonged period in a prone position and the application of an emergency response belt across the face resulting in asphyxia.
Mr Orchard, who was prescribed anti-psychotic medication, worked as a cleaner at his local church, St Thomas's, in Cowick Street, Exeter.
He was described as a quiet and introverted man who had suffered outbreaks of aggressive behaviour because of his illness but was not known to bite, spit or threaten.
Prior to his arrest, members of the public in Exeter city centre immediately thought it was "obvious" he was suffering from mental health problems, the jury heard.
Five police officers and two police community support officers arrested Mr Orchard, who was 5ft 7ins and weighed just over 12 stone.
He was handcuffed with restraints around his upper and lower legs then taken to the custody unit where Kingshott, Tansley and Marsden were on duty.
After appearing to bite an officer, Mr Orchard was held down while restrained at three points and controlled by three officers - with four immediately beside him.
Tansley asked: "Can you get the ERB for us?" and the fabric restraint device was fetched by another detention officer.
Tansley placed the belt around Mr Orchard's face before giving the command "one, two, three, up". Mr Orchard was then lifted to waist height in the prone position.
None of the four people who lifted him had sole control of his head. One was lifting him with one hand and holding his head in the ERB with the other, it was alleged.
"It was wrong," Mr Heywood said.
He said the officers had not been trained to use the belt in that way and it carried a "strikingly obvious" risk of obstructing Mr Orchard's airway.
"He can't talk to you, who knows if he can breathe. And if he can't, how can he communicate?
"That's exactly what happened here. It's the start of events that led to Mr Orchard's death," he said.
The trial, expected to last for up to nine weeks, continues.