Inadequate planning and communication were what led to the 2016 gas pipeline explosion and fire in Alabama that killed two workers and injured four others, according to an investigation by the National Transportation Safety Board (NTSB).

The October 31 blast near Helena occurred when a contractor who was excavating damaged the Colonial Pipeline Company’s (Colonial) 36-inch diameter refined liquid petroleum transmission pipeline, known as Line 1. The damage resulted in a release of gasoline from the pipeline, which ignited and burned for several days. Two excavation crew workers died, and four other workers were injured.

The NTSB determined that the probable cause of the explosion was the excavation crew’s inadequate planning, coordination, and communication during the excavation and failure to adhere to company policy requiring hand excavation if closer than two feet from the top or bottom of the pipeline until the pipeline has been exposed, which allowed the track-hoe to damage the pipeline.