A medical examiner found that a young black man suffered a "high-energy injury," most likely caused when the Baltimore police van he was riding in suddenly slowed down.
A post-mortem report obtained by The Baltimore Sun newspaper says Freddie Gray's death could not be ruled an accident and is instead a homicide because officers did not follow safety procedures "through acts of omission".
Police arrested Mr Gray, 25, on April 12 and he died a week later, prompting protests and rioting. A grand jury indicted six officers on various charges. One officer faces the most serious charge of second-degree "depraved-heart" murder. They have pleaded not guilty.
The Baltimore Sun reported that the post-mortem found the injury, similar to those suffered in shallow-water diving, was most likely caused when the van suddenly decelerated.
A spokesman for the Maryland medical examiner and for the prosecutor's office declined to release the report, but State's Attorney Marilyn Mosby condemned the leak.
"I want to make it very clear that the state's attorney's office did not release the Freddie Gray autopsy report. As I have repeatedly stated, I strongly condemn anyone with access to trial evidence who has leaked information prior to the resolution of this case," she said.
Lawyers for the officers released a joint statement saying they had not yet received the report, although Ms Mosby is expected to turn it over to the defence by Friday.
Although officers loaded Mr Gray into the van on his abdomen, the medical examiner surmised he may have got to his feet, then been thrown into a wall when the van abruptly changed direction.
Because Mr Gray was not belted in and had his wrists and ankles shackled, he was "at risk for an unsupported fall during acceleration or deceleration of the van".
Police and a lawyer for the Gray family have said previously that Mr Gray suffered a severe spine injury.
At the University of Maryland Medical Centre, Mr Gray tested positive for opiates and cannabinoid, according to the post-mortem.
According to the report's chronology:
:: Mr Gray suffered no injuries to suggest a neck hold or anything stemming from physical restraint during his arrest. Assistant medical examiner Carol Allan noted that in bystander video, Mr Gray is seen bearing weight on his legs and speaking as officers load him into the van.
:: The van made several stops. At the second stop, officers placed an identification band and leg restraints on Mr Gray. "Reportedly, Mr Gray was still yelling and shaking the van. He was removed from the van and placed on the ground in a kneeling position, facing the van doors, while ankle cuffs were placed, and then slid onto the floor of the van, belly down and head first, reportedly still verbally and physically active." The most significant injury to Mr Gray may have occurred after this stop.
:: During a fourth stop, authorities said van driver Caesar Goodson called for help and Officer William Porter got involved. "The assisting officer opened the doors and observed Mr Gray lying belly down on the floor with his head facing the cabin compartment, and reportedly he was asking for help, saying he couldn't breathe, couldn't get up, and needed a medic. The officer assisted Mr Gray to the bench and the van continued on its way."
:: When the van made a fifth stop to pick up a second arrestee, "Mr Gray was found kneeling on the floor, facing the front of the van and slumped over to his right against the bench, and reportedly appeared lethargic with minimal responses to direct questions."