DECATUR, Ga. -- A man fatally shot by a police officer was "unarmed, unclothed and unable to do harm," a prosecutor told jurors Thursday. But a defence attorney repeatedly stressed that the officer had only seconds to react as the man ran at him.

The jury heard opening statements Thursday in the trial of Robert "Chip" Olsen. The former DeKalb County police officer faces charges, including felony murder, in the March 2015 death of 26-year-old Anthony Hill.

Olsen sat at the defence table with his hands clasped and his head facing forward during the opening statements.

Prosecutor Buffy Thomas talked about Hill and Olsen and then walked them through what happened on the day of the shooting.

Hill joined the Air Force in 2008 and served just under five years before he was medically discharged after he was diagnosed with bipolar disorder and PTSD, Thomas said. Olsen, now 57, had been a police officer for seven years, she said.

Around 1 p.m. on March 9, 2015, leasing office staff at the apartment complex where Hill lived saw him lying on the ground outside wearing only shorts. He got up and knocked on the door of the leasing office.

Two maintenance workers returning from lunch saw Hill behaving strangely and tried to get him to go home. Hill said he needed help, that the devil was coming and that he loved his mother. One of the maintenance workers helped him home, but Hill reemerged a short time later completely naked.

Hill wasn't being aggressive, but he was hanging on railings and doing a military crawl on the playground, Thomas said. When one of the maintenance workers tried to get him to go home and said the police were coming, Hill told him, "Good, the police are my friends."

The leasing staff office had called 911 three times as this was playing out.

When Olsen drove into the complex, he looked through a gap between buildings and saw Hill crouching on the ground. Olsen continued around a bend so that his patrol car was facing Hill.

Hill jumped up and began running toward the car. Olsen exited his vehicle, drew his gun and shouted, "Stop! Stop!" Hill didn't stop running, and Olsen fired twice when Hill was about 5 to 7 feet away.

Olsen told the second officer on the scene that Hill had attacked him and pounded on his chest. But Thomas told jurors that witnesses said Hill never touched or threatened Olsen. Under questioning by Georgia Bureau of Investigation agents the next week, Olsen "never, ever said he was afraid."

"He wasn't afraid," Thomas said. "He was uncomfortable with this naked man running at him. That's what it was."

Olsen's use of deadly force was excessive and he violated department policy and his oath of office, Thomas said.

Defence attorney Don Samuel told the jurors that while they may wish by the end of the trial that Olsen had reacted differently, they will not be able to conclude that he's guilty of murder.

"This case boils down to all of about 6 or 7 seconds that Chip Olsen had to react to what any person would view as being an imminent use of violent force against him," Samuel said.

Olsen didn't know that Hill was a resident of the complex or that he suffered from mental illness, Samuel said. What he knew was there had been three 911 calls reporting a possibly demented person, he said.

Hill tried to deescalate the situation and pulled his gun to gain control, not to shoot, Samuel said. But Hill didn't stop running even as Olsen yelled to stop and took steps backward.

"I just want you to think for a second how frightening that must be, how frightening it was for Chip Olsen," Samuel said.

Samuel rejected suggestions that Olsen should have used one of the other tools he had available, like a stun gun, baton or pepper spray. None of those work well when someone is approaching quickly, he said.

A grand jury indicted Olsen nearly a year after the shooting on charges of felony murder, aggravated assault, violation of oath of office and making a false statement. Olsen resigned following the indictment.

A felony murder charge doesn't mean prosecutors believe Olsen acted with malice but rather that he killed someone while committing another felony, in this case aggravated assault or violation of his oath.

News outlets, including The Associated Press, have previously reported Hill was 27. His mother and prosecutors said Thursday that he was 26 when he died.