Robert Bates is charged with second-degree manslaughter for the death of Eric Courtney Harris. If convicted, he faces up to four years in prison.

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Robert Bates was a volunteer deputy when he shot and killed Eric Courtney Harris last year

Bates' defense: He meant to use his Taser but accidentally pulled and fired his gun instead

Internal investigation: Bates was given preferential treatment at the sheriff's department

CNN  — 

An Oklahoma jury starts deciding this week whether an ex-sheriff’s deputy should go to prison for shooting a man in the back.

Robert Bates, an ex-volunteer reserve sheriff deputy for the Tulsa County Sheriff’s Office, never denied killing Eric Courtney Harris last year. But he said he meant to use his Taser stun gun, not his revolver.

The all white jury, consisting of six women and eight men, began hearing opening statements in the second-degree manslaughter trial Wednesday. If convicted, Bates could face up to four years in prison.

‘Oh! I shot him. I’m sorry!’

On April 2, 2015, an undercover deputy was conducting a sting operation to try to catch Harris illegally selling a gun.

Bates, 73, was parked several blocks away. He told NBC’s “Today Show” he had participated in several hundred sting operations but always in a backup role, where he would “clean up” after deputies and take photos and notes after they had made an arrest.

But officials said as deputies rolled up to arrest Harris that day, the suspect bolted and ran toward Bates. A tussle ensued. Bates fired his gun into Harris’ back.

‘Oh! I shot him! I’m sorry!” Bates said, as captured in a video of the shooting.

His defense attorney, Clark Brewster, said experts will testify about the effects of the stress Bates was under as he made the mistake of drawing his gun and not his Taser that day.

How easy (or hard) is it to confuse a gun for a Taser?

Inside the courtroom

Prosecutor Kevin Gray began in a dramatic fashion. Gray repeated Harris’ words in the video after Harris sold an undercover officer a gun, then began to run away from police in a hot pursuit.

“He’s running, he’s running, he’s running,” the prosecutor quoted the officer in the video, then clapped his hands to make the sound of the gunshot. “From the time Eric Harris climbed into a truck and didn’t know deputy Lance Ramsey was an undercover officer. It took 10 minutes. He had no way of knowing he’d be dead,” Gray said.

“You’re gonna hear this gun was used by Mr. Bates,” the prosecutor said, and held up the gun that is part of the evidence. “You’ll hear him say Taser” but you will never see that Taser leave his vest.”

Courtroom sketch from right to left: Defendant Robert Bates, assistant district attorney Kevin Gray, District Judge William J. Musseman, Jr., defense lawyer Clark Brewster.

The defense emphasized the dangers of undercover police operations, arguing Harris’ character was “dangerous” and in the situation “everyone was on high alert and filled with stress.” Bates’ attorney frequently called Harris by his nickname “40,” suggesting to the jury Harris could have been a gangster who “made it known he’s a Rollin’ 90’s crip,” Brewster said.

Bates was the only person with a Taser, the defense said. Brewster told the jury that both the Taser and the gun were similar in weight, had the same look and feel and similar laser on them. “When he yelled Taser, Taser, Taser. He mistakenly had his gun instead of his Taser,” Brewster said.

The first witness to take the stand on Wednesday, was the undercover officer whose hands and voice are on video as he bought the gun from Harris in the undercover sting.

Fallout at the Sheriff’s Office

Shortly after the shooting, critics questioned Bates’ qualifications as a volunteer deputy – and wondered whether his close personal friendship with then-Sheriff Stanley Glanz helped get him preferential treatment.

At the time of the shooting, Bates was a CEO of an insurance company who volunteered as a sheriff’s deputy.

Tulsa shooting casts spotlight on volunteer police programs

An internal inquiry by the Tulsa County Sheriff’s Office in 2009 found that Bates was shown special treatment and that training policies were violated during his time there.

A grand jury indicted the sheriff in September on two misdemeanor charges, including one saying Glanz “denied lawful requests for the release of internal investigations into his office’s Reserve Deputy program.”

After almost 30 years as Tulsa County’s sheriff, Glanz resigned.

CNN’s Azadeh Ansari and Shawn Nottingham contributed to this report.