(Reuters) - A white Tulsa police officer charged with killing an unarmed black motorist took the stand in her own defense on Monday, saying she fired because training taught her to act quickly against a perceived deadly threat, according to local news reports.

Betty Shelby, 43, could face between four years and life in prison if convicted of manslaughter in the September 2016 killing of Terence Crutcher.

Lawyers for Shelby have said she believed that Crutcher, who was 40, may have been trying to reach through a partially open window for a weapon in the vehicle when she shot him.

"If you hesitate, if you delay, then you die," Shelby was quoted in a Tulsa World news story as telling the state district court.

Police videos of the incident were seen globally, and some civil rights advocates have argued that race was a factor in the fatal shooting, something Shelby has denied. Rights advocates saw the Crutcher case as another in a string of deaths of unarmed black men at the hands of the police in the United States that has spawned periodic protests and the Black Lives Matter movement.

In the videos, Crutcher can be seen with his hands in the air shortly before he was shot. Tulsa police have said Crutcher was unarmed and there was no weapon in his vehicle, which was blocking a road.

Shelby told the jury that she was taught during law enforcement training that if a suspect reaches into an area like a car, an officer does not let them pull their arm back because they might be holding a gun, the Tulsa World reported from the court room.

She said the first time she fired her weapon on duty was when she shot Crutcher, who did not speak during their encounter, according to court testimony from officers on the scene.

Prosecutors have said there was no reason for Shelby to fire on a man who was walking away from her. They blame her for turning a routine traffic matter into a deadly confrontation by acting unreasonably and escalating the situation.

The Oklahoma Medical Examiner's Office said that Crutcher had 96 nanograms per milliliter of the hallucinogenic drug PCP in his bloodstream at the time of his death.

Prosecutors have previously said Crutcher’s drug use was not reason enough for Shelby to resort to deadly force, local media reports said.

(Reporting by Lenzy Krehbiel-Burton in Tulsa and Jon Herskovitz in Austin, Texas; Editing by Frances Kerry)