ST. LOUIS - Justin Biancardi, a former volunteer police officer in Uplands Park, admitted Monday in federal court here that he helped a full-time officer shake down more than a dozen prostitutes for money in 2009.

The other officer, Leon F. Pullen, has admitted sexually assaulting four of the women and asking Biancardi to cover for him. Biancardi, who cooperated with authorities, could face up to 10 years in prison and a $250,000 fine at sentencing April 13.

Meanwhile, more details emerged in the case of Pullen, who had 10 of 19 felony charges against him dismissed when he pleaded guilty in July to the assaults. His sentencing is scheduled for Jan. 28.

In a rambling, handwritten letter to the court Sept. 1, Pullen says he is guilty only of some of the charges.

"I did not sexual assault or kidnap or rob any female sir. These women are lying on that part. I never touch them sir I have never, ever touch a woman in no bad way ever sir," he wrote.

The letter was classified by the court as a motion to withdraw his plea. Prosecutors, in opposing it, used it as an opportunity to further detail a case in which they previously described only a July 2009 encounter with 36-year-old woman called D.S.

They elaborated on the assaults of the three other women that year after Pullen responded to escort-service ads.

Officials described an encounter at a St. Louis hotel room, where Pullen displayed a gun and forced a woman to have sex or face arrest. At a St. Louis gasoline station, they said, Pullen met two escorts and molested one in his patrol car and stole her money. At an Uplands Park gas station, Pullen allegedly told a woman she was being arrested, and he molested her. Prosecutors say Pullen took a cell phone and $600.

In the case of D.S., Pullen and Biancardi arranged to meet the woman and another escort in Uplands Park and arrested them, officials said. At the station, Pullen allegedly forced D.S. to sit on a bulletproof vest and forcibly performed oral sex upon her.

Pullen, later suspended from the force, denies that assault in his letter, saying "somehow she got my DNA in which I have no clue sir but I tell you sir I did not touch her in any way."