The National Transportation Safety Board has released a Pipeline Accident Brief on TransCanada Corporation's Keystone Pipeline Rupture in Amherst, South Dakota on November 16, 2017. The NTSB concluded that the probable cause of the pipeline failure was a fatigue crack, likely originating from mechanical damage to the pipe by a tracked vehicle during pipeline installation.
In addition to the safety issues raised, this report emphasizes that the NTSB may accept jurisdiction and investigate releases and other pipeline accidents, even in the absence of death, personal injury or widespread environmental damage. Preparing for an NTSB investigation is critical for all pipeline operators.
The complete report and accident docket can be viewed here.