BALTIMORE — A judge Monday found the highest-ranking officer charged in Freddie Gray’s arrest and death not guilty on all counts, dealing a devastating blow to prosecutors, who have now tried four of the six officers initially indicted without winning a conviction.

Judge Barry G. Williams acquitted Lt. Brian Rice, 42, of manslaughter, reckless endangerment and misconduct in office after a week-long bench trial, finding that Rice didn’t commit a crime when he loaded Gray into a police transport van without seat-belting him. Gray suffered a fatal neck injury as he was being taken to a police station.

The verdict renews questions about whether the state should move forward with charges against the remaining officers or drop them altogether. Two of Rice’s co-defendants were recently acquitted, and a third is awaiting retrial after a jury deadlocked in his case in December. Another two officers are due to be tried in the coming months.

Police arrested Gray in West Baltimore on the morning of April 12, 2015, after he ran from officers on bike patrol. Rice and other officers shackled his wrists and legs and put him in the prisoner compartment of a police van without restraining him. Prosecutors say he fell and struck his head. He died a week later.

Gray’s death became a flashpoint in a roiling national debate over racial profiling and the deaths of black men in police custody. Peaceful demonstrations in Baltimore gave way to riots and looting in the days after the 25-year-old’s funeral, prompting officials to impose a citywide curfew and call in the National Guard to restore order. State’s Attorney Marilyn Mosby announced charges against the officers in a nationally televised news conference, saying she would seek justice for the city’s young people.

Rice’s trial played out against a backdrop of national tension and mourning over recent police shootings of black men in Minnesota and Louisiana and the racially motivated shootings in Dallas that killed five police officers this month.

Prosecutors sought to show that Rice, as the senior officer on the scene, bore responsibility for Gray’s death. They said he was well aware of department policy requiring officers to seat-belt all detainees but chose to ignore it. In closing arguments, prosecutor Janice Bledsoe said Rice’s actions were deliberate and that the lieutenant wanted to “punish and humiliate” Gray for resisting arrest.

Defense attorneys said Gray’s “belligerent” behavior that morning made it too dangerous for officers to climb into the van’s cramped prisoner compartment and seat-belt him.