Owners of the Keystone pipeline knew a defect had formed years before the strain finally caused the pipeline to burst and flood a Kansas creek with oil last year. 

A third-party review of a pipeline spill that released 500,000 gallons of crude oil onto Kansas farmland and a nearby stream was caused by a crack in the metal pipe that eventually ruptured under pressure.

The Keystone oil pipeline was operating at the bounds of its permit when it burst and released almost 13,000 barrels of oil in northern Kansas, an executive from the company said Tuesday.

Regulators want to know the risks that flawed welding or shifting ground could pose for more breaks on the Keystone pipeline, which has spilled repeatedly since 2011.

Canadian oil company TC Energy said Thursday that faulty welding contributed to an “instantaneous rupture” in its Keystone pipeline that gushed hundreds of thousands of gallons of extra sticky tar sands crude oil onto Kansas native prairie, cropland and into a creek.

Hundreds of workers have been hustling around the clock to recover oil for two months after the Keystone pipeline spill. Some landowners want more information about the cleanup and about why the pipeline broke.

The pipeline company that spilled nearly 600,000 gallons of oil onto fields and into a stream in north-central Kansas says it has cleaned up more than 85% of the crude.

When the Keystone pipeline burst in rural Kansas last month, county workers rushed to build an emergency dam on Mill Creek. Federal agencies dispatched pipeline and environmental experts, and the state set about sampling water and searching for injured animals. All of this costs taxpayers.

Owners of the Keystone oil pipeline must complete their cleanup of a massive spill in northern Kansas under oversight of the federal government, the Environmental Protection Agency announced Monday.