A Maryland judge has acquitted Baltimore police Lieutenant Brian Rice of involuntary manslaughter, reckless endangerment and misconduct in office 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.
Rice, 42, is the highest-ranking officer charged in Mr Gray's death from a broken neck suffered in a police transport van.
Mr Gray's 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 Louisiana.
Monday's verdict is the latest setback for prosecutors, who have failed to secure a conviction in the trials of four officers thus far.
Lt Rice, who is white, ordered two officers on bicycles to chase Mr Gray, 25, when he fled unprovoked in a high-crime area.
Prosecutors said Lt Rice was negligent in shackling Mr Gray's legs and not securing him in a seat belt, as required by department protocol.
But defence lawyers said Lt 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 Mr Gray was being combative and a hostile crowd was looking on, they said.
Williams, who heard the case without a jury at Lt 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.