The focus of the limited investigation was a metallurgical evaluation of the ruptured pipe. Materials engineers in the NTSB Materials Laboratory used optical and scanning electron microscopes and energy dispersive spectroscopy to examine the fracture surface and adjacent exterior pipe surfaces to determine the mode of fracture. The NTSB determined the probable cause of the failure of the Keystone Pipeline was a fatigue crack, likely originating from mechanical damage to the pipe exterior by a metal-tracked vehicle during pipeline installation. The fatigue crack grew and extended in-service to a critical size, resulting in the rupture of the pipeline.