A judge has acquitted a police van driver of all charges including "depraved-heart" murder in the death of 25-year-old arrestee Freddie Gray, whose broken neck on the way to the station set off Baltimore's worst riots in decades.

Six officers were charged in Gray's death, but only Officer Caesar Goodson was accused of murder.

Gray was fatally injured after officers bound his hands and feet and Goodson left him unprotected by a seatbelt that prosecutors say would have kept him from slamming into the van's metal walls.

Baltimore Circuit Judge Barry Williams delivered his verdict on Thursday after hearing five days of testimony in the non-jury trial.

He also found Goodson not guilty of manslaughter, assault, misconduct in office, and reckless endangerment.

READ MORE: Justice, Baltimore style

Williams ruled that the state failed to show that Goodson knew he had caused harm to Gray by leaving him unbuckled, or that Goodson was aware that Gray was injured and intentionally failed to call a medic.

"The state failed to prove the defendant knew or should have known that Mr Gray needed medical care," the judge said.

"Unlike in a shooting or a stabbing, or a car accident, this injury manifests itself internally," Williams said, citing conflicting testimony from medical experts. "If the doctors weren't clear, how would a person without medical training know?"

The acquittal of Goodson, 46, is perhaps the most significant blow to State Attorney Marilyn Mosby's efforts to hold police accountable for Gray's death.

Protests and rioting

Last month, the same judge acquitted Officer Edward Nero of misdemeanour charges, and in December, he declared a mistrial after a jury failed to agree on manslaughter and other charges against Officer William Porter.

Porter faces a retrial in September, and three other officers have yet to be tried.

Protests and rioting after Gray's death on April 19, 2015 set the city on fire, forcing Maryland to bring in the National Guard. The unrest forced the city's mayor to abandon her re-election campaign, and the Department of Justice opened an investigation into allegations of widespread police abuse.

Prosecutors said Goodson was criminally negligent when he failed to buckle Gray into a seatbelt or call for medical aid after Gray indicated that he wanted to go to a hospital.

But Goodson would not talk to investigators or take the stand at trial, leaving the state with slim evidence of intent to harm.

Source: Agencies