A Cleveland cop is on trial over a chase that ended with officers shooting at an unarmed couple 137 times

The trial of a Cleveland police officer charged with killing an unarmed couple during a police chase started this week.

Michael Brelo, 31, is one of several officers who was involved in the 2012 chase that led to the deaths of Timothy Russell and Malissa Williams, who were shot at 137 times while parked in a middle school parking lot.

The Ohio attorney general released a report saying the car chase and ensuing shooting was the result of "systemic failure in the Cleveland Police Department," according to The Plain Dealer in Cleveland.

Thirteen total police officers were involved in the shooting, but prosecutors say that Brelo was the only one who continued shooting at the couple even after other cops had decided they weren't a threat, according to The Plain Dealer. An investigation found that Brelo jumped onto the hood of the couple's parked car and shot directly down at them.

Others within the Cleveland police department were disciplined over the incident, but Brelo is the only one who has been charged with manslaughter.

Brelo faces 25 years in prison if he's convicted, according to ABC News.

Here's what happened:

Police at the time called the shooting a "full blown-out" firefight.

Ohio Attorney General Mike DeWine told reporters in 2013 that "there is nothing normal about this case. ... This is a tragedy."

In November 2012, about 60 police vehicles pursued the two suspects in a 25-minute chase spanning three cities. One suspect, 30-year-old Malissa Williams, was shot 24 times, and the other, 43-year-old Timothy Russell, was shot 23 times.

Williams and Russell were both homeless and probably driving around trying to buy drugs, according to a report from the Bureau of Criminal Investigations. A plainclothes officer spotted their car in an area known for drug deals and called in the license plate.

When the officer tried to pull Russell over for a turn signal violation, he sped away, The Plain Dealer reports. Other officers later joined in the chase.

At the time, police said the suspects fired shots at them near Cleveland's downtown Justice Center, and a police dispatcher said that shots were fired at officers during the chase, according to The Plain Dealer.

Investigators later determined that the "shot" was likely Russell's car backfiring.

The chase ended in a gunfight near Heritage Middle School, and when police checked the car they realized neither Williams nor Russell was armed.

The attorney general's report shows that the officers likely believed the suspects were armed based on erroneous information broadcast over the police radio. Officers also told investigators they saw the suspects in the car with what looked like a gun.

Brelo told investigators that he thought the officers were being shot at and that Russell would run them over.

During the chase, Russell was reportedly high and driving on a suspended license. Toxicology results show that he was under the influence of cocaine and alcohol at the time of his death. Williams was also high on cocaine and had marijuana in her system.

The officers being disciplined for violating police protocol won't lose their jobs and did not partake in the gunfight at the end of the chase, but they did have a role in the pursuit. More than 100 police officers were involved in the chase in some way.

Five police supervisors have been indicted on charges of negligence of duty.

Brelo pleaded not guilty to two charges of voluntary manslaughter.

In 2011, The Plain Dealer published an investigation of reports of excessive use of nondeadly force by Cleveland police officers. The newspaper found that the police chief often overlooked inconsistencies with police officers' stories when investigating use of force incidents.

Many of the officers faced accusations of brutality on the force.