Baltimore police officer Edward Nero was acquitted on Monday of all charges in the 2015 death of black detainee Freddie Gray, an incident that triggered rioting and protests and fuelled the Black Lives Matter movement.
Baltimore City Circuit Court Judge Barry Williams, who heard the case in a bench trial, issued the verdict before a packed courtroom. Nero, 30, had faced misdemeanour charges of second-degree assault, reckless endangerment and two counts of misconduct in office.
There were no initial signs of rioting after the verdict but a group of protesters chased members of Nero's family into a parking garage, yelling, "No justice, no peace."
Mayor Stephanie Rawlings-Blake issued a statement urging calm and said Nero would face an administrative review by the police department.
"In the case of any disturbance in the city, we are prepared to respond," she said.
Nero had been charged with arresting Gray, 25, without justification in April 2015 and failing to secure him in a police van, where he suffered a fatal spine injury.
Williams said prosecutors had failed to prove their case. During a 25-reading of his decision, he said that Nero acted as a "similarly situated" officer would.
He said Nero's partner, Garrett Miller, had testified that Nero had not handcuffed Gray.
Gray's death a week after his arrest sparked a day of rioting in which nearly 400 buildings were damaged or destroyed in the majority black city of 620,000 people. The case helped stoke the Black Lives Matter movement and national debate over policing in minority communities.
Nero was among six officers charged in Gray's death and the second to go to trial. The trial of the first officer to be tried, William Porter, ended in a hung jury in December.
Nero's lawyers had argued that Gray's arrest was justified and that the officer had little to do with it. He never touched Gray except when he tried to help him find an asthma inhaler and helped lift him into the van once he was shackled, they said.
The hashtag #FreddieGray began trending on Twitter in the United States after news of Nero's acquittal and some black activists on Twitter expressed their disappointment.
"#FreddieGray should be alive today," wrote DeRay Mckesson, a key figure in the Black Lives Matter movement who unsuccessfully ran for mayor of Baltimore in April.
"With yet another clearance of an officer in the death of #FreddieGray - the justice system completely fails us," tweeted Shaun King, a columnist for the New York Daily News.