A Cleveland police officer has been found not guilty over the shooting of two unarmed suspects in a 137-shot barrage of police gunfire after a high-speed chase.

The judge's verdict on 31-year-old Michael Brelo comes after a four-week trial on two counts of voluntary manslaughter over the deaths of Timothy Russell and Malissa Williams on November 29, 2012.

Thirteen officers fired at the suspects' car that night in a school car park, yet only Brelo was charged criminally.

Prosecutors said he waited until the vehicle had stopped and the occupants were no longer a threat to step onto the bonnet of the car and fire 15 rounds into the windshield.

Brelo could have faced 22 years in prison if convicted on both counts.

The lead attorney for Brelo said the prosecution spared no expense and "were ruthless".

Patrick D'Angelo called the case a "tragedy" that was brought about by the actions of the two people who died.

He said Brelo risked his life the night Russell and Williams led officers on a long police chase through the city's streets.

The head of the city's police union said Brelo was held accountable through the indictment, trial and ultimate acquittal.

Steve Loomis of Cleveland Patrolemen's Association said he hoped the community respects the judge and the process.

But after the verdict at least 30 protesters gathered at the Cleveland court house.

About an equal number of sheriff's deputies bearing clear shields stood in front of the building as the demonstrators chanted "hands up, don't shoot".

Later the Justice Department said it planned to "review all available legal options" after the acquittal.

Officials said they will review the trial testimony and evidence to determine if "additional steps are available and appropriate" in the federal judicial system.

The department said the review was separate from its efforts to resolve a pattern of civil rights violations at the Cleveland police department. A report in December outlined a string of examples of excessive force, including officers who unnecessarily fired guns, hit suspects in the head with weapons, and punched and used Tasers on people already handcuffed.

Judge John P O'Donnell, who found Brelo not guilty on all charges, concluded that the patrolman was justified in using lethal force. Judge O'Donnell also said it could not be determined who fired the fatal shots.

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