A post-mortem examination obtained by the Baltimore Sun has found that Freddie Gray suffered a “high-energy injury” inside a police van.
Gray, 25, died in hospital a week later, sparking riots in Baltimore and protests across the US about the use of lethal force by police.
Gray’s injuries were similar to those suffered in shallow-water diving, the report found.
Six Baltimore police officers have been charged in the case.
The driver of the police van, Caesar Goodson, faces the most serious charge – of second-degree murder.
The officers have pleaded not guilty and the trial is expected start in October.
The prosecutor in the case, Marilyn Mosby, condemned the leaking of the report, saying trial evidence should remain confidential.
The state medical examiner’s report found Gray’s death was not an accident because officers did not properly secure Gray inside the van.
It said that Gray was “at risk for an unsupported fall during acceleration or deceleration of the van”.
After Gray’s arrest on 12 April, officers shackled his wrists and ankles and placed him inside the van on his stomach.
The authors of report surmised that Gray got to his feet and was eventually thrown into a wall when the van abruptly changed direction.
Gray was arrested after he fled from police officers. Many including Baltimore Police Commissioner Anthony Batts have questioned why Gray was detained in the first place.