A former assistant to R. Kelly testified on Friday she once saw him engage in sexual activity with one of the women he is charged with abusing, as prosecutors neared the end of presenting their sex trafficking case against the R&B singer.

On the 18th day of testimony at Kelly’s trial in federal court in Brooklyn, Cheryl Mack, the mother of music producer London on da Track, said she saw the woman begin to massage Kelly while backstage at a Connecticut concert where he was performing.

“That was kind of my cue to leave,” Mack said. “I was very uncomfortable.” She said that as she left the room she caught a glimpse of the woman moving her head toward Kelly’s groin.

Mack also said Kelly lost his temper in 2015 after she supposedly ruined a surprise birthday party for former stylist Kash Howard, and had her sign an “apology letter” that included false claims she accepted kickbacks from booking agents.

“I apologized out of fear,” Mack said.

Several witnesses have testified that Kelly made them write apology letters, purportedly to absolve him of misconduct, but which prosecutors could use to illustrate the tight control that witnesses have said Kelly demanded over those around him.

Friday’s final prosecution witness was Dawn Hughes, a clinical psychologist who discussed how people might groom underage girls for sex, and how victims cope with the trauma. Defense lawyers are expected to cross-examine her on Monday.

Kelly, 54, has pleaded not guilty to charges over his alleged grooming and preying on women and girls as far back as the mid-1990s, when he shot to stardom with music including the 1996 smash “I Believe I Can Fly.”

The singer, whose full name is Robert Sylvester Kelly, faces one count of racketeering and eight counts of illegally transporting people across state lines for prostitution.

Kelly’s indictment said he abused six women and underage girls including the singer Aaliyah, who Kelly married illegally in 1994 when she was 15. Aaliyah died in a 2001 plane crash.

Prosecution witnesses have said Kelly instilled fear as he demanded they follow his stringent rules, including by having women and girls call him “Daddy,” and punished those who disobeyed, including by demanding unwanted sex.

Kelly’s lawyers have tried to portray his accusers as fans who once hoped to capitalize on the singer’s fame but felt jilted, and questioned why they and former employees failed to leave Kelly sooner or go to the police.

Deveraux Cannick, one of Kelly’s lawyers, tried while cross-examining Mack to show jurors she should have known not to sign an apology letter, given that she was in her late 40s and had many years of music industry experience.

“I wasn’t thinking at all,” Mack told him.

The trial began on August 18. Kelly has faced sexual abuse accusations for nearly two decades.

Scrutiny increased after the #MeToo movement began in late 2017, and Lifetime aired the documentary Surviving R. Kelly in January 2019.

Kelly also faces sex-related charges in Illinois and Minnesota.