The pair were chatting on the MySpace social networking website.
Kreigsman said the girl - who lives in New York - notified her local sheriff's office, which contacted authorities in North Carolina.
Investigators say they found a computer and DVD with attack plans at the 15-year-old's home, but said those plans were not elaborate.
Kreigsman said the teen is in custody and undergoing a mental-health evaluation. He declined to identify the teen, noting that no charges have been filed.