A Baltimore Police inquiry has found that the van transporting Freddie Gray made a previously undisclosed stop while en route to the police station.
Freddie Gray suffered fatal and unexplained spinal injuries while in police custody, sparking two weeks of protests that turned violent earlier this week.
On April 30, there were rallies in Baltimore, Philadelphia and Cincinnati.
A national debate over the use of lethal police force has been going on since the death of teenager Michael Brown in Ferguson, Missouri, last summer.
Police investigating Freddie Gray’s death said they found out about the new van stop from a security camera.
Deputy Police Commissioner Kevin Davis said: “We discovered this new stop based on our thorough and comprehensive and on-going review of all CCTV cameras and privately owned cameras.”
The new video was filmed by a CCTV camera outside a small Korean food shop.
The shop’s owner, Jung Hyun Hwang told the Associated Press news agency that police officers visited last week to make a copy of the recording – which was later lost when the shop was looted during the riots.
Jung Hyun Hwang said he had not viewed the recording and did not know what it showed.
Investigators have now handed over their inquiry into Freddie Gray’s death to the state’s attorney’s office.
Baltimore’s top prosecutor, Marilyn Mosby, will now decide whether to take the case to a grand jury to seek an indictment of any of the six officers involved.
After two nights of violent protests in Baltimore, Thursday was relatively calm. Baltimore is still under a curfew requiring people to be off the streets by 22:00.
Visiting Baltimore, Maryland Governor Larry Hogan said the city was “not out of the woods yet”.
“There are a lot of people that have legitimate frustrations that are peacefully protesting and we want to protect those people and their right to express their feelings,” he said.
“But we’re also concerned about their safety because there are other people who just want to cause trouble.”
Freddie Gray was injured when arrested in Sandtown on April 12. He lapsed into a coma and died a week later.
Mobile phone video from a bystander shows two officers dragging Freddie Gray into the van by the arms.
According to the police timeline of the arrest, the van took 30 minutes to take him to the police station, where paramedics were called.
While in the van, Freddie Gray was requesting medical attention which he was wrongly denied, police have admitted.
They also acknowledged that Freddie Gray was not secured in the van by a seatbelt, which contravenes department policy.
A local ABC station, quoting unnamed sources, said the medical examiner has concluded that Freddie Gray received his injuries inside the van, not when he was first arrested.
Previously, police had said the van made three stops, including one to put him in leg irons and another to pick up another prisoner. The new stop makes four in total.
Five of the six officers involved in the arrest gave statements to investigators the day Freddie Gray was injured. All six have been suspended.
A separate investigation by the US Department of Justice is also under way.