WASHINGTON (AFP) - The most senior Baltimore police officer to face trial for the death of Freddie Gray was acquitted of all charges Monday (July 18), in a case that sparked riots and fuelled a national debate over police treatment of black Americans.

Local media reported the verdict handed down by a judge in the case of Brian Rice, 42, the fourth of six Baltimore police officers to go on trial for Gray's death.

The first three trials ended in two other acquittals and one hung jury, raising questions of whether anyone will ever be punished for Gray's death, which police say was an accident.

Gray, 25, was arrested on April 12, 2015 after fleeing at the sight of police, and suffered a broken spine while being transported in the back of a Baltimore police van, unsecured and with his hands and feet bound. He died a week later.

Rice was tried on charges of involuntary manslaughter, second-degree assault, misconduct in office and reckless endangerment.

Two other police officers have yet to be tried in Gray's death. Of the six officers charged in the racially charged case, three are white and three are black.

The trial was held with the United States roiling over more recent deaths of black men at the hands of police, as well as apparent reprisal killings of police, including the fatal shooting in Baton Rouge, Louisiana on Sunday of three police officers.

That shooting, by a black former Marine who had served in Iraq, took place in a city scarred by racial tensions and protests after the July 5 death of Alton Sterling, a black man shot at point-blank range by two white Baton Rouge police officers.