Baltimore's top police officials, mayor and prosecutor sought to calm a "community on edge" Monday while investigating how a man suffered a fatal spine injury while under arrest. Six officers have been suspended, but investigators say they still don't know how it happened.

A week after the arrest of Freddie Gray, police said they still don't have any video or other evidence explaining what happened to cause the "medical emergency" that an arresting officer said Gray suffered while being taken to the local police station, Deputy Commissioner Jerry Rodriguez said.

Gray was black; the race of the officers involved has not been disclosed. His death came amid a national debate about the deaths of black men at the hands of police.

Autopsy results returned Monday show that Gray suffered "a significant spinal injury that led to his death," Rodriguez said. "What we don't know is how he suffered that injury."

The Gray family's lawyer, Billy Murphy, had said that Gray's "spine was 80 per cent severed at his neck."

Police released a more detailed timeline of how Gray was arrested and transported on April 12. It revealed that Gray was placed in leg irons after an officer felt he was becoming "irate," and that the van stopped on its way to the police station, even picking up another prisoner in an unrelated case, while Gray repeatedly asked for medical attention.

Police Commissioner Anthony Batts said that Gray asked first for an inhaler, and then several times during his transport for medical care.

"There were several times he made a medical request," Batts said. "He asked for an inhaler, and at one or two of the stops it was noticed that he was having trouble breathing and we probably should have asked for paramedics."

Something must have happened between the time Gray was videotaped by a bystander being dragged into the van, and the time he arrived at the station in deep distress, the deputy commissioner said.

"When Mr. Gray was put in that van, he could talk, he was upset. And when he was taken out of that van, he could not talk and he could not breathe," Rodriguez said.

Mayor Stephanie Rawlings-Blake said she too is "angry that we are here again" after trying to overcome decades of distrust between the black community and police in Baltimore's inner city.

Batts said he is ordering that police review and rewrite "effective immediately" its policies on moving prisoners and providing them with medical attention.

"We are a community on edge right now. We hear, I hear, the outrage. I hear the concern and I hear the fear," Batts said, asking for calm. "We are on edge as a city, and I need your help to make sure we get this out in the proper way."

Officer Garrett Miller's official request for a criminal charge against Gray, a 25-year-old black man who was only 5-foot-8 inches tall and 145 pounds, said that he had been arrested "without force or incident."

Miller sought a charge of carrying a switchblade, punishable by a year in prison and a $500 fine, according to court records obtained Monday by The Associated Press.

Miller's charging document doesn't provide any explanations for the injuries that would lead to Gray's death a week later. He wrote only that while being taken to the station, on April 12, "the defendant suffered a medical emergency and was immediately transported to Shock Trauma via medic."

Another 30 minutes passed before police finally called an ambulance to pick Gray up at the station. He arrived at the hospital in critical condition and died on Sunday after a weeklong coma.

The documents, which misspell Gray's name as "Grey," were first reported Monday by The Baltimore Sun. Police had not previously mentioned a knife, or publicly disclosed the charge against Gray.

Activists protesting excessive use of force and even Baltimore city officials say they have more questions than answers. About 50 people marched from City Hall to police headquarters Monday, carrying signs reading "Black lives matter" and "Jobs, not police killings." They unfurled a yellow banner reading "Stop police terror."