OKLAHOMA CITY (Reuters) - Closing arguments are planned for Monday in the trial of a former Oklahoma City police officer accused of raping and sexually assaulting 13 women while on duty, threatening many with arrests if they did not perform sexual acts.

Daniel Holtzclaw, 28, of Oklahoma City is charged with 36 counts of sexual assault, including six first-degree rape counts. He could face life in prison if convicted of the most serious charges.

Thirteen of his suspected victims took the stand in the trial that began more than a month ago, telling jurors of the sexual assaults that ranged from touching over the clothes to forced oral sex and rape.

On Wednesday, the prosecution presented its last scheduled witness, a DNA expert who testified about Holtzclaw’s uniform pants testing positive for DNA from a 17-year-old girl.

The girl, now 18, testified Tuesday that Holtzclaw raped her on her mother’s front porch.

Holtzclaw appears to have targeted his victims by going after women he came across on patrol. He would run background checks and go after women who had outstanding warrants, previous arrests or carried drugs or drug paraphernalia, prosecutors said.

He did this because he did not think authorities would take their word over his if he had to defend himself against sexual assault allegations, they added.

On Wednesday, the defense attorneys presented their only witness, Holtzclaw’s ex-girlfriend Kerri Hunt, who said she dated him for a year starting in March 2014. She testified that Holtzclaw never made her feel uncomfortable and described his sex drive as “normal for someone his age.”

Holtzclaw declined to testify in his defense. His lawyers have denied the accusations and said any DNA linked to him and the women came through non-sexual contact.

Holtzclaw is mixed race white and Asian, and all of his victims were African-American. The trial comes as a series of incidents across the United States has raised questions of racial bias in U.S. policing.

Holtzclaw was fired in January 2015 after about three years on the job.