Faulty welding and bending stress fatigue on TC Energy's Keystone pipeline caused the largest oil spill in nearly a decade, the pipeline operator announced Thursday last week.

According to ABC news, TC Energy—the pipeline's Canada-based operator, said the cause was established through an independent lab analysis report on the 2,687-mile section of the conduit, which remained shut down after the disastrous leak in December 2022.

Last year's accident on TC Energy's pipeline in rural Washington County, Kansas, resulted in a spill of more than 500,000 gallons of oil spill, which cost $480,000, according to TC Energy.

"Although welding inspection and testing were conducted within applicable codes and standards, the weld flaw led to a crack that propagated over time as a result of bending stress fatigue, eventually leading to an instantaneous rupture," TC Energy stated.

The company reported that the faulty weld in a fitting girth, connecting two sections of pipe, was completed at a fabrication facility, with the cause of the pipeline's bending stress still under investigation.

According to the company, the pipeline typically pumps over 26 million gallons of oil daily from Alberta, Canada, to refineries in Texas, Illinois, and Oklahoma. The metallurgical analysis revealed that the pipe and manufactured fitting had no issues with strength or material properties, and the pipeline was operating within its operational and design's maximum operating pressure.

The leak is the most recent in a series of accidents on the pipeline, with a federal report released in 2021 revealing that the conduit experienced 22 accidents between 2010 and 2020, with the severity of spills worsening in recent years.

The report, conducted by the U.S. Government Accountability Office, discovered that the previous incidents spilt a total of 11,975 barrels of crude oil or a little over 500,000 gallons.

According to the report, four of the largest Keystone Pipeline oil spills between 2010 and 2020 were due to issues related to the original design, pipe manufacturing, or pipeline construction.