ATTLEBORO - A Bristol County assistant prosecutor at Attleboro District Court has been charged with sexually assaulting a 17-year-old girl at the courthouse last Friday.

Pete Joseph Costanza, 39, who supervises other assistant district attorneys in the Attleboro office, was being held on $10,000 cash bail at the Attleboro police station pending arraignment today.

The teen told court officers and court personnel Friday afternoon that she had been sexually assaulted by Costanza in a conference room adjacent to the district attorney's office at the courthouse, police said.

The area is closed to the public and is used by lawyers to conference cases or by the staff of the district attorney's office to talk to victims and witnesses.

The victim was involved in a criminal case but no information was immediately released as to what her role in the case or what kind of case it was. Costanza was arrested without incident Sunday at his home after police obtained a warrant for his arrest, police said.

Detectives Lt. John Otrando and Russell Castro and Joseph Collins, a state police detective with the Bristol County District Attorney's Office, investigated the allegations through the weekend.

Police said they interviewed several people and collected evidence during the course of the investigation.

Costanza has been an assistant district attorney since April 2005 under Paul Walsh, the previous Bristol County district attorney.

He was a prosecutor at New Bedford District Court before being appointed by Bristol County District Attorney Sam Sutter to be the supervising assistant district attorney in the Attleboro office in April.

Costanza was a lawyer in Pueblo, Colo., and has been a lawyer for six years in Massachusetts.

He faces charges of indecent assault and battery, assault and battery and attempted extortion.

Police contacted Sutter and a decision was made to have a special prosecutor from the Plymouth County District Attorney's Office to handle the case.

Attleboro police referred calls to the Plymouth and Bristol county district attorney's offices.

Gregg Miliote, a spokesman for Sutter, said the district attorney will have the Plymouth County District Attorney's Office handle the case "because the district attorney's office, obviously, cannot prosecute one of its own employees." Costanza has been suspended without pay pending an internal investigation.