Cleveland officials said Tuesday they are firing six police officers involved in a 137-shot barrage that killed two unarmed black people after a high-speed chase.

Those officers included Michael Brelo, a patrolman acquitted of manslaughter charges in May for having fired the last 15 shots of the barrage in East Cleveland on Nov. 29, 2012. The chase began when officers standing outside police headquarters mistook the sound of a beat-up Chevrolet Malibu backfiring as a gunshot.

Six more officers who fired during the barrage face suspensions ranging from 21 to 30 days, said Public Safety Director Michael McGrath, the former police chief. A total of 13 officers had been notified they faced administrative discipline, and one of them has retired, McGrath said.

The president of Cleveland's largest police union, Steve Loomis, vowed to fight the discipline and said grievances appealing the firings were filed with the city Tuesday. He described the firings as unbelievable, unprecedented and politically motivated.

"It's tragic that it went down this way, but at the end of the day, two people high on crack cocaine, high on marijuana, one of them intoxicated, made the decisions that they made and we responded to them," Loomis said. "And we responded within our training."

Cleveland police officer Michael Brelo looks on during his manslaughter trial in on April 6, 2015. Brelo, who was acquitted, was fired Tuesday. (Tony Dejak/Reuters)

The officers had been cited for joining the chase or leaving the city without permission. More seriously, some were accused of endangering other officers by creating or contributing to a crossfire situation.

McGrath said officials reviewed hundreds of pages of related documents and multimedia to reach decisions on discipline.

"It was very difficult. ... But at the end of the day, there were general police orders. There are manual rules and regulations that we expect officers to comply with," McGrath said. "If they didn't comply with those particular general police orders or manual rules and regulations, I sustained charges."

Victims each hit by more than 23 shots

The high-speed chase involved 62 police cruisers and more than 100 officers. The shooting killed both occupants of the car. Timothy Russell was hit by 24 shots, Malissa Williams by 23.

Authorities never learned why Russell didn't stop. He had a criminal record including convictions for receiving stolen property and robbery and had been involved in a previous police pursuit. Williams had convictions for drug-related charges and attempted abduction. Both were described as mentally ill, homeless and addicted to drugs. A crack pipe was found in the car.

Of the officers who fired, only Brelo faced criminal charges. Prosecutors said he stood on the vehicle's hood and fired inside repeatedly after the car had stopped and its occupants were no longer a threat.

Defence attorneys said the officers involved in the chase and shooting had probable cause to believe the people in the car were a safety threat. Brelo's disciplinary letter cites that and also notes that he fired 49 times, more than double any other officer involved.

Investigators eventually concluded Russell and Williams weren't armed.

Loomis said the officers were right to chase the car because Russell fled at a high rate of speed. After the car stopped, the first shots were fired after Russell used the car as a deadly weapon and drove at an officer, Loomis said.

The shooting preceded a monthslong U.S. Department of Justice investigation that concluded Cleveland police engaged in a pattern and practice of using excessive force and violating people's civil rights. The city negotiated an agreement to make changes overseen by an independent monitor.

It also paid a $3 million US settlement in a lawsuit filed by relatives of Russell and Williams.

Dorothy Sigelmier, Williams' aunt, told cleveland.com she felt "OK" about the officers' discipline but wished they had ended up in jail. She said she's forgiven them.