By Donna Owens

BALTIMORE (Reuters) - Baltimore prosecutors on Monday failed for the fourth time to secure a conviction against a city police officer for the death of black detainee Freddie Gray, as a lieutenant was cleared of all charges.

The acquittal of Lieutenant Brian Rice renews questions about the prospects for the remaining cases stemming from the death of Gray, who suffered a fatal neck injury in April 2015 after he was bundled into the back of a police transport van.

Baltimore's police union called on prosecutors to drop the charges against three officers still awaiting trial in the case. Gray's death triggered protests and rioting in the mainly black city and stoked a national debate about how police treat minorities.

Tensions flared anew this month with the deaths of African-American men at the hands of police in Minnesota and Louisiana. The controversy took a tragic turn when eight police officers were shot dead in apparent reprisal attacks staged by lone black gunmen in Dallas and Baton Rouge, Louisiana.

Rice, 42, the highest-ranking officer charged in the Gray case, was acquitted of involuntary manslaughter, reckless endangerment and misconduct following a bench trial.

Baltimore City Circuit Court Judge Barry Williams, who oversaw a non-jury trial at Rice's request, said prosecutors did not prove that Gray died as a result of Rice's failure to secure him with a seat belt.

In separate statements, Baltimore Mayor Stephanie Rawlings-Blake and Congressman Elijah Cummings, who represents the district Gray lived in, urged city residents to respect Williams' decisions.

Baltimore was peaceful after the verdict. Only a handful of protesters had gathered outside the downtown courthouse and there were no police reports of unrest.

Rice was the fourth of six officers to stand trial in the case. Williams had acquitted Officers Edward Nero and Caesar Goodson Jr., both of whom were in court on Monday.

Goodson, the driver of the van, had faced the most serious charge, second-degree murder. Officer William Porter faces a September retrial after a jury deadlocked.

Officer Garrett Miller is scheduled for trial this month, while Sergeant Alicia White's trial is set for October. Porter and White face manslaughter among their charges, while Miller is charged with assault and other crimes.

Warren Alperstein, a Baltimore defense attorney who attended the trial as a spectator, said he was "not surprised by the verdict whatsoever."

"At the end of the day, the state may have to say we're cutting our losses and moving on," he said.

But Doug Colbert, a law professor at the University of Maryland, said there was still value in having brought the prosecutions, even if they were unsuccessful.

"The police departments are now on notice that the legal community stands ready to prosecute in these types of cases," he said.

Prosecutors and defense lawyers in the case are barred from commenting by a gag order from Williams.

Rice, who is white, ordered two officers on bicycle to chase Gray, 25, when he fled unprovoked in a high-crime area.

Prosecutors said Rice acted negligently by failing to secure Gray with a seat belt in the van.

But defense lawyers said Rice made a reasonable split-second decision while Gray was being combative and a hostile crowd looked on, they said.

The judge said prosecutors failed to show the lieutenant was aware of a departmental policy requiring seat belts for prisoners during transport.

"A mere error in judgment is not enough to show corruption," Williams said.

(Writing by Ian Simpson in Washington and Joseph Ax in New York; Editing by Jeffrey Benkoe and Tom Brown)