A Maryland judge has acquitted Baltimore police Lieutenant Brian Rice of involuntary manslaughter for the April 2015 death of black detainee Freddie Gray.

Baltimore City Circuit Court Judge Barry Williams handed down his verdict on Monday after a bench trial, also acquitting Rice of reckless endangerment and misconduct in office.

Rice, 42, is the highest-ranking officer of the six charged in Gray’s death, who suffered a broken neck in a police transport van.

The death triggered protests and rioting in the mainly black city and stoked a national debate about how police treat minorities.

The controversy flared anew this month with the deaths of African-American men at the hands of police in Minnesota and Baton Rouge.

Monday’s verdict is the latest setback for prosecutors, who have failed to secure a conviction in the trials of four officers thus far.

Rice, who is white, ordered two officers on bicycles to chase Gray, 25, when he fled unprovoked in a high-crime area.

A mural depicting Freddie Gray seen past blighted row homes in Baltimore at the intersection where Gray was arrested. Photo: AAP

Prosecutors said Rice was negligent in shackling Gray’s legs and not securing him in a seat belt, as required by department protocol.

But defence lawyers said Rice was allowed leeway on whether to get inside a van to secure a prisoner. The officer made a correct decision in a few seconds while Gray was being combative, and a hostile crowd was looking on, they said.

Williams, who heard the case without a jury at Rice’s request, said prosecutors failed to show the lieutenant was aware of a departmental policy requiring seat belts for prisoners during transport.

“The state did not prove the defendant was aware of the new policy,” the judge said in court.

Only a handful of protesters were at the courthouse for the verdict’s announcement.

Williams previously acquitted Officers Edward Nero and Caesar Goodson Jr., the van’s driver. A third officer, William Porter, faces a retrial after a jury deadlocked.