A preliminary investigation has found that the 210,000-gallon oil spill last week from the Keystone pipeline was caused by damage sustained when the line was constructed in 2008.

The Department of Transportation’s Pipeline and Hazardous Materials Safety Administration (PHMSA) outlined its initial conclusion in an order released late Tuesday that allowed TransCanada Corp. to return the line to operation nearly two weeks after the South Dakota leak was reported, but under lower pressure, among other conditions.

“The rupture has characteristics of mechanical damage from original construction. Preliminary information indicates the failure may have been caused by mechanical damage to the pipeline and coating associated with a weight installed on the pipeline in 2008,” Alan Mayberry, PHMSA’s associate administrator for pipeline safety, wrote in the order.

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TransCanada, which is planning to build the controversial Keystone XL pipeline to expand oil capacity heading south from Alberta, Canada, removed the damaged portion of pipe and replaced it. It is being sent to a National Transportation Safety Board lab for metallurgical testing.

Crews are still working to clean up the oil that spilled from the underground pipe.

In the meantime, however, PHMSA is allowing Keystone to restart operations. The agency approved TransCanada’s plan to restart with an initial pressure about 20 percent below its usual level at the affected area, reducing the line’s capacity.

PHMSA is also requiring TransCanada to prepare a more detailed remedial action plan from the incident and an analysis of the root causes of it.