Lisa Van Allen, who was a witness in the R&B singer's child pornography trial, said she and Kelly made a sex tape with Kelly's underage goddaughter.
Van Allen said one of Kelly's associates paid her to return the tape after she stole it out of his duffel bag and tried to blackmail him.
Kelly was acquitted last week of child pornography charges in connection with a separate tape prosecutors said showed him having sex with his goddaughter.
The Chicago Tribune said law enforcement officials now plan to investigate Van Allen's allegations about the existence of another tape showing Kelly had a sexual relationship with the young teen.
Assistant State's Attorney Shauna Boliker, who was the lead prosecutor in Kelly's trial, told the Tribune she couldn't discuss the allegations because an office policy prohibits prosecutors from commenting on cases unless charges have been filed.
Sources told the newspaper authorities are to concentrate on whether Kelly's employee
bought the tape and concealed it, since the evidence may have supported the prosecution's case against Kelly.
Kelly's associate has denied doing anything wrong, the Tribune said.