The police officer who fatally shot an unarmed 40-year-old black man in Tulsa, Oklahoma, last week is formally facing a manslaughter charge.

Tulsa County District Attorney Steve Kunzweiler announced on Thursday that a warrant had been issued for the arrest of Officer Betty Shelby. The officer, who joined the Tulsa Police Department in 2011, fatally shot Terence Crutcher on Sept. 16, according to authorities.

Kunzweiler said arrangements are being made with her lawyer for Shelby to turn herself in to authorities.

"Officer Shelby, although now charged, is presumed to be innocent until a judge or a jury determines otherwise," Kunzweiler emphasized while announcing the warant. "I do not know why things happen in this world the way they do. We need to pray for wisdom and guidance on each of our respective paths in life. Each of us at the end of the our days will have to accoount for our own actions. The only way I know how to walk my path is to try everyday to pray and to serve my fellow citizen so that he or she may be lifted up."

Under Oklahoma law, a person can face a first-degree manslaughter charge if he or she kills someone "unnecessarily, either while resisting an attempt by the person killed to commit a crime or after such attempt shall have failed," among other circumstances.

If convicted, Shelby could face a minimum of four years in prison.

Mayor Dewey Bartlett praised the police department for its quick work in a statement released shortly after Kunzweiler's announcement.

"I appreciate their efforts as well as the district attorney's usual thorough evaluation of the rules of law for which we are all accountable," he said. "These are important steps to ensure that justice and accountability prevails."

He promised further transparency on behalf of investigators.

"Whether through peaceful demonstrations, prayer vigils or countless statements of support to the Crutcher family and the entire community, we are in this together as one Tulsa," he said. "We continue to pray together, mourn together and we will get through this together."

Shortly after the charges were announced, Gov. Mary Fallin issued a statement, in which she said she hoped the decision to bring charges would bring some peace to the Crutcher family, while reminding Oklahomans that Shelby is innocent until proven guilty.

Crutcher's death sparked nationwide outcry amid a rash of recent, fatal police-involved shootings.

Video released by police showed Crutcher walking away from Shelby with his hands in the air. The footage does not offer a clear view of the single shot that killed Crutcher.

Shelby was the first to respond to the scene Friday when someone reported that Crutcher's vehicle was blocking traffic, according to police. Shelby found Crutcher and his car in the middle of the road. Family members later said his car was having issues and stalled as he was headed home from a local community college.

Officers said Crutcher refused to follow police commands. Shelby's attorney has said Crutcher was not following police commands and that Shelby opened fire when the man began to reach into his SUV window.

The Associated Press contributed to this report.