A judge in Baltimore Monday acquitted the highest-ranking officer charged in the death of Freddie Gray, marking the fourth trial that prosecutors failed to win.

Lt. Brian Rice faced charges of involuntary manslaughter, reckless endangerment and misconduct in office. He opted for a bench trial by Baltimore Circuit Court Judge Barry Williams.

The judge previously dismissed a second-degree assault charge, and prosecutors dropped a second misconduct charge against the 42-year-old officer, who is white.

After the verdict announcement, Baltimore Mayor Stephanie Rawlings-Blake tweeted that Rice would face an administrative review by the police department. She asked for the community's continued respect for the judicial process.

Gray died a week after suffering a spinal injury in a police van last year, touching off protests and rioting.

Rice's failure to seatbelt Gray, Williams said, may have been an error in judgment and a violation of updated policy -- but the judge found prosecutors failed to prove it rose to a criminal level.

"The state failed to show that the defendant, even if he was aware of the risk, consciously disregarded that risk," the judge said.

Three earlier trials resulted in two acquittals and a mistrial.

Officers Caesar Goodson and Edward Nero -- both of whom were found not guilty in bench trials -- were also present at the courthouse.

Gray was arrested after he ran from police officers in a high-crime area of the city. He was handcuffed and later shackled in the back of the police van, but officers never buckled him in.

Prosecutors had said Rice was most responsible of the six officers charged for following police procedures to fasten a prisoner in a seat belt, citing his 18 years of experience on the force.

The officer's attorney said police could use discretion, if they believe their safety is at risk. Rice attorney Michael Belsky said officers had concerns because Gray was not cooperative and they weren't sure what onlookers would do if extra time was taken to fasten Gray in the van.

Prosecutors and defense attorneys gave different characterizations of the onlookers. Prosecutors described them as concerned observers, while Belsky said officers heard threatening comments during the arrest.

Three of the charged officers are black, and three are white. Race has not been cited as a direct factor in Gray's death, but his arrest and deadly injury added momentum to the Black Lives Matter movement.

Gray's family received a $6.4 million settlement from the city.

Baltimore State's Attorney Marilyn Mosby vowed to bring justice to an aggrieved citizenry when she announced the charges last year. But she has yet to find success in court, and is being sued for defamation by five of the officers.

Fox News' Varuna Bhatia and The Associated Press contributed to this report.