One person was killed and another was injured in a Southeastern Clarion County Traffic Collision, very early this Thursday morning.     State Police identify the deceased as 21-year-old Stephanie Mongelli of the Brookville area.     It was shortly before 12:30 a.m. that Mongelli was driving a car on Route 28, just north of Settlers Road, in Redbank Township.     For unknown reasons, the Mongelli vehicle crossed the centerline of Route 28.     The car collided, head-on, with an oncoming truck tractor, which was operated by Jose Mejia of New York.     Mongelli was pronounced dead, at the scene, by Clarion County Coroner Terry Shaffer.     The truck driver was injured and State Police say that he was taken to Brookville Hospital for treament.     Route 28, at the wreck site, was closed for three hours after he collision.     Also aiding, at the scene, were Summerville and Pinecreek Fire-Rescue.