A Pennsylvania prosecutor says a 17-year-old girl was "the prime instigator" in the torture and slaying of a mentally disabled woman last year.

Westmoreland County District Attorney John Peck says the teen, Angela Marinucci, and five adults were casual friends of the victim, 30-year-old Jennifer Daugherty.

Daugherty's body was found in February 2010 in a school parking lot in Greensburg, Pa. She had been bound and stabbed.

Peck says Marinucci was jealous of Daugherty and instigated a brutal attack. He is seeking a first-degree murder conviction for the now-18-year-old defendant.

Defense attorney Michael DeMatt told the jury during his opening statement Monday that it's unlikely a high school student like Marinucci could have persuaded a group of adults to commit the "ultimate act" of murder.