Complex sentence: Susie's body was mangled and entangled in pieces of the seat from the waist down and this led investigators to believe that the bomb was positioned underneath her seat (the passenger side seat of Fritz's Chevrolet Trailblazer).
Split into simpler sentences: Susie's body was mangled from the waist down and many pieces of the seat were entangled in her corpse. This led investigators to believe that the bomb was positioned underneath her seat (the passenger side seat of Fritz's Chevrolet Trailblazer).