A major oil spill from TC Energy Corp.’s Keystone crude pipeline last year was caused by fatigue crack, according to the company.

When the pipeline was constructed, a crack formed in a segment of the pipe that was welded to a fitting, TC Energy said in an analysis it submitted to federal regulators. Over time, the crack led to a rupture. The company is implementing a plan to avoid future spills that includes investigating other sites that could be at risk, it said. 

A section of the massive pipeline, which can carry more than 600,000 barrels a day, suffered its worst-ever leak on Dec. 7. The conduit was halted for more than three weeks, roiling crude markets and limiting supplies to the storage hub at Cushing, Oklahoma, the reference point for US oil prices. More than 13,000 barrels spilled, with some of the oil entering a local creek.

Keystone has leaked more oil than any other pipeline in the US since 2010, spilling more than 25,000 barrels, according to Pipeline and Hazardous Materials Safety Administration data. The conduit’s first phase was commissioned in 2010.

TC’s analysis comes as environmental and community groups have staunchly opposed pipeline projects on the grounds that structural integrity can’t be guaranteed, increasing the threats to water quality and human health. The report could also prompt a regulatory crackdown by the Biden administration, which has already demonstrated deep skepticism of the industry’s track record.