Cincinnati's mayor was forced to impose an overnight curfew to curb violent protests after a white police officer was acquitted of causing the death of an unarmed black man.
The unrest, in which protesters pelted cars with rocks and bottles and lit small fires, happened in the same neighbourhood that saw three days of rioting when Officer Stephen Roach shot Timothy Thomas on 7 April.
Police said Wednesday night's unrest was not nearly as bad as the rioting last spring and had quickly died down. Twelve adults and two juveniles were arrested overnight on charges of curfew violations, disorderly conduct and drug offences.
Thomas, who was wanted on 14 warrants including traffic charges and previously fleeing police, had been shot in the chest by Mr Roach, 27, after running from three other officers.
Mr Roach was acquitted on charges of negligent homicide and obstructing official business. About 12 hours later, the violence erupted. The mayor, Charlie Luken, declared a state of emergency. Several cars were hit with rocks and bottles about one block from where a vigil was held for Thomas, who was 19. One car was set on fire, and there were at least 30 fire department runs to put out rubbish bin fires, police said.
Peaceful protests had been held earlier outside the courthouse and at City Hall after the Hamilton County municipal judge, Ralph Winkler, cleared Mr Roach. Judge Winkler, who heard the case without a jury at Mr Roach's request, concluded that the shooting was "not a culpable criminal act" because Mr Roach believed he had to shoot or be shot.
"I would give anything to change the outcome of what happened that night, but unfortunately I can't," Mr Roach – who faced nine months' jail if convicted on both charges – said outside the courtroom.
In last spring's riots, dozens were injured and more than 800 arrested. Cincinnati had not seen such racial unrest since Martin Luther King's assassination in 1968.