Shortly after a white police officer in the US state of Ohio was found not guilty in the killing of an unarmed black man and woman after a 22-mile car chase in November 2012, US Justice Department announced that it would review the case.

"We will now review the testimony and evidence presented in the state trial. We will continue our assessment, review all available legal options and will collaboratively determine what, if any, additional steps are available and appropriate given the requirements and limitations of the applicable laws in the federal judicial system," the federal agency said in a statement.

After a month-long non-jury trial, a judge earlier Saturday found Cleveland Officer Michael Brelo, 31, not guilty of two counts of felony voluntary manslaughter in the deaths of Timothy Russell, 43, and Malissa Williams, 30.

"The state did not prove beyond a reasonable doubt that the defendant Michael Brelo knowingly caused the deaths of Timothy Russell and Malissa Williams," announced Cuyahoga County judge John O'Donnell.

Judge O'Donnell also ruled that Brelo was not guilty of two other lesser counts of felonious assault.

Brelo was one of the 13 officers who fired up to 137 shots at Russell and Williams after the car chase on Nov. 29, 2012.

The car chase began after Russell's 1979 Chevy Malibu backfired while driving past local police headquarters. Mistaking the sound for gun shot from the car, up to 62 police patrol cars took part in the car chase till the vehicle of the suspects rammed a police car in a school parking lot in East Cleveland, police said.

Thirteen officers, including Brelo, then fired more than 100 times in less than 10 seconds at the car. However, Judge O'Donnell said this round of police gunfires was justified because the policemen had reason to believe safety of themselves and the public was at risk.

None of the other 12 officers were charged in the incident.

According to prosecutors, Brelo then exited his car and climbed onto the suspects' car hood and fired 15 shots down through the windshield into the victims when the suspects were no longer deemed as a threat, a main reason why only him was criminally charged.

Defense attorney previously claimed that Brelo was fearful for his safety, a notion obviously shared by the judge.

"Brelo reasonably perceived a threat," said the judge, adding that evidence does not prove that Brelo's shots were the ones which killed the pair.

Before Saturday's verdict, the killings of Russell and Williams were highlighted by a 2014 Justice Department investigation report into policing practices in Cleveland Police Department, which found that a pattern of abusive policing had long been existing.

"Our investigation concluded that there is reasonable cause to believe that CDP (Cleveland Police Department) engages in a pattern of practice of using unreasonable force in violation of the Fourth Amendment," said the report, citing unnecessary use of deadly force and excessive or retaliatory use of less lethal force.

Meanwhile, emotions immediately ran high outside the court room after the announcement of acquittal, with protesters chanting "No justice, no peace," an exclaim repeatedly heard in rallies across the US cities following police killing of unarmed black men.

Cleveland mayor Saturday afternoon called for calm from protesters.

"It is my expectation that we will show the nation that peaceful demonstration and dialogue is the right direction as we move forward as one Cleveland," said mayor Frank Jackson.

According to Jackson, the city was working closely with other law enforcement agencies to keep tabs on any possible rioting that might arise in the wake of Saturday's verdict.

The possibility of another mass demonstration in a US city came less than one month after rioting convulsed Baltimore in the state of Maryland following the death of a black young man in police custody.