Freddie Gray's fatal spinal injury was caused when he slammed into the back of a Baltimore police van, a local ABC affiliate reports.

A head injury he sustained matches a bolt in the back of the wagon, according to the report, which cites unnamed multiple law enforcement sources.

The preliminary investigation also found no evidence the 25-year-old was hurt during his videotaped arrest, reports WJLA.

It is not clear from the report if Mr Gray's injury was sustained inside the wagon during the journey to the police station. Mr Gray died on 19 April, a week after his arrest.

Dozens of protesters went on the march through downtown Baltimore on Thursday hours after police said they had turned over their internal investigation to a prosecutor.

Police revealed their inquiry had uncovered a previously unknown stop by the van which took Mr Gray to the station.

It was captured by a camera outside a small grocery store.

Shop owner Jung Hyun Hwang said his original recording of the CCTV was on a laptop that was stolen during the city's riots on Monday night, but police had been able to obtain a copy.

Before Mr Gray - who was allegedly found to be carrying a switchblade - was put in the van, he asked officers for an inhaler, but did not receive one.

At the first stop, he was put in leg cuffs because police said he became "irate".

The second stop, which has only just come to light, took place at the junction of N Fremont Avenue and Mosher Street.

The van stopped a third time, and the driver asked for an additional unit to check on Mr Gray.

At a fourth stop, the wagon picked up another prisoner.

The prosecutor who must now decide whether to charge six police officers involved in the arrest asked the public on Thursday to remain patient.

State's Attorney Marilyn Mosby, 35, is the youngest top prosecutor of any US city.

Police also said on Thursday that 98 police officers had been injured since the violence that erupted after Mr Gray's funeral on Monday.

The Washington Post cites a police document as stating that Mr Gray was "intentionally trying to injure himself" while in the back of the wagon.

According to the newspaper, a prisoner who was also in the van told investigators that Mr Gray was "banging against the walls" of the vehicle.

The newspaper said it was not clear if evidence backed up the claims of the prisoner, whose identity it did not disclose.

Protests against police brutality spread to other major cities on Wednesday for the first time since Mr Gray died on 19 April.

Thousands of National Guard troops and police remain on the streets of the Maryland city, which is under overnight curfew.