Tulsa, Oklahoma (CNN) A jury found a sheriff's deputy guilty of second-degree manslaughter Wednesday in the fatal shooting of an unarmed suspect.

Robert Bates, who was a volunteer reserve sheriff deputy for the Tulsa County Sheriff's Office last year at the time of the shooting, never denied shooting Eric Courtney Harris.

Bates, 74, said he meant to use his Taser stun gun, not his revolver, on the suspect, who had been tackled by other deputies and was being held on the ground.

The jury deliberated less than three hours and recommended Bates serve four years in prison, the maximum possible sentence. Preliminary sentencing is set for May 31. After the verdict, Bates was escorted out of the courtroom by two deputies from the department he once served.

In his closing argument to the jury, defense lawyer Clark Brewster said Bates should be be thanked for trying to help his fellow deputies. He displayed the stun gun and Bates' pistol and showed how they were similar size and weight.

"He got out of his vehicle to man up and help," Brewster said. "I truly believe you will find this was an accident driven to this point by the actions of Mr. Harris."

Robert Bates

Prosecutor John David Luton told the jury Bates was nodding off in his car before the arrest. He said it was wrong to blame Harris for the shooting.

"Bob Bates didn't act with usual and ordinary care," Luton said in his closing argument. "He also didn't do what a reasonable person would do under similar circumstances. ... Eric Harris deserved to be chased, he deserved to be tackled, he deserved to be arrested. He did not deserve to be killed by reserve deputy Bob Bates."

Bates was CEO of insurance company

The death of Harris is one of several nationally known cases in which a white law enforcement officer killed an unarmed black man. These cases have galvanized the public over tactics that police are trained to use.

The all-white jury, consisting of six women and eight men, had to decide if the shooting was, in the words of court charging documents, "an act of culpable negligence." Those documents said Bates believed he had his Taser in his hand when he shot.

Bates didn't testify in his own defense and only made one comment on the record in court. When the judge asked if he was satisfied with his defense, Bates replied, "I'm absolutely tickled to death."

Eric Courtney Harris.

At the time of the shooting, Bates was 73 and the CEO of an insurance company. He had worked for the Tulsa Police Department for a year in the 1960s and had been a reserve deputy since 2008.

On April 2, 2015, he was providing backup and parked several blocks away from an undercover officer conducting a sting operation to try to catch Harris illegally selling a gun.

As deputies rolled up to arrest Harris that day, the suspect bolted and was pursued by officers, who caught him and took him to the ground. Bates got out of his vehicle and fired his pistol into Harris' back.

Bates experienced heavy stress, psychiatrist says

'Oh! I shot him! I'm sorry!" Bates said, as captured in a video of the shooting.

Authorities said Bates thought he pulled out his Taser but "inadvertently" fired his gun.

The defense put on two expert witnesses who were both medical doctors. They testified Harris died of a heart attack not from he bullet that pierced his lungs. But the Forensic pathologist Cheryl Niblo testified that Harris' death was a homicide caused by a gunshot.

One of the things the prosecution had to prove to get a guilty verdict was whether Harris was killed by Bates when Bates shot him.

The first defense witness was Dr. Charles Morgan, a forensic psychiatrist, who was shown video of the incident. He testified that Bates may have pulled his gun during the arrest because people tend to resort to their habits, not training, during times of "uncontrollable stress."

JUST WATCHED 2015: Robert Bates speaks about shooting Replay More Videos ... MUST WATCH 2015: Robert Bates speaks about shooting 01:17

Prosecutor Kevin Gray questioned Morgan about how he knew it was Bates' habit to use his pistol. After being prodded by the judge, Morgan answered that he didn't look at any of Bates' training records to come to his conclusion.

Bates' training was an issue throughout the investigation and trial.

Deputy Ricardo Vaca testified that if Bates actually had used a stun gun instead of a pistol at the moment the shot was fired, it would not have been consistent with their training.

"You are supposed to wait until you have a clear opening and then deploy," he said.

Vaca was the first deputy to tackle Harris and was on top of him when the shooting happened.

'I almost got killed,' deputy says

"I almost got killed," Vaca said, his voice cracking. "It makes me emotional. Inches to my right and I would have been killed,"

Vaca and another deputy testified they observed Bates in his patrol vehicle nodding off a few minutes before the takedown order was given.

Deputy Michael Heisten said Bates gave a statement to investigators and claimed to have been in situations like this before. He meant to use nonlethal force as he had in the past, the statement said, according to Heisten.

"Based on his record how often had Bates been in a situation involving a fleeing felon?" Gray asked.

"Never." Heisten replied.

The defense contended that mistaking the stun gun for the pistol was an honest mistake. On video, Bates could be heard saying he was going to deploy his stun gun. Deputy Leighton Boyd testified he heard Bates say that and moved back to avoid being hit by a stun gun prong.

The judge allowed Brewster, the defense lawyer, to give the jury an opportunity to hold Bates' gun and a stun gun similar to the one Bates carried that day. Brewster took the revolver himself and activated the barrel laser on the courtroom wall. The stun gun also projects a red dot, he said.

Heisten, a detective with the sheriff's office, said the weapons are different. A switch must be flipped before making the stun gun operational. There is no corresponding switch on the revolver, Heisten said.

'I thank God for the conviction'

After court convened, a tearful Andre Harris, brother of Eric Harris, he was relieved the jury convicted Bates.

"I thank God for the conviction," Andre Harris said. "I hope he's [Bates] taught a lesson that all lives matter."

Bates should never have been working as a deputy, he said.

"For a 73-year-old to be out on a drug task force, supposedly chasing deadly criminals, is not his line of work," Harris said. "Seventy-three is the age where you retire with your grandkids and enjoy life."

Andre Harris also said "pay for play" helped Bates obtain his position as a reserve deputy. The Harris family lawyer has said Bates donated equipment to the sheriff's department, an allegation the sheriff's department has denied.

An internal inquiry by the Tulsa County Sheriff's Office in 2009 concluded that Bates was shown special treatment and that training policies were violated regarding his role with the agency.

Information about the internal investigation was not revealed to the jury.