Tesoro Logistics LP detected anomalies during an inspection of its 20-year-old North Dakota pipeline just days before the line ruptured and spilled 20,600 barrels of oil onto farmland, the company said.

The six-inch pipeline was carrying Bakken oil to the Stampede rail facility outside Columbus, ND, when it ruptured. A farmer harvesting wheat discovered oil spouting from the line Sept. 29.

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A robot, known as a “smart pig,” detected anomalies during what Tesoro called routine internal inspections of the pipeline Sept. 10 and 11.

“We were awaiting results of the analysis of that inspection when the leak was reported,” Tesoro spokeswoman Tina Barbee said.

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North Dakota regulators said corrosion on the pipeline may have caused the spill.

The U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration was on site investigating the spill despite the government shutdown over the last few weeks.

The pipeline remains down while the federal regulator looks into how Tesoro responded to the spill, its control-room processes and records and whether non-compliance contributed to the pipeline’s failure, said Jeannie Shiffer, PHMSA’s director for Governmental, International, and Public Affairs.

The ruptured pipeline runs 35 miles from Tioga to Black Slough in North Dakota. BP Plc built the pipeline in 1993.

It is a part of Tesoro’s “High Plains” pipeline system in North Dakota and Montana that gathers oil from the Bakken shale and delivers it to another pipeline and to Tesoro’s 68,000 barrels-per-day Mandan refinery.

Texas-based Tesoro bought the pipeline and the refinery from BP in 2001.

The spill is the largest on U.S. soil since an Exxon Mobil pipeline spilled 5,000 to 7,000 barrels of heavy Canadian crude in Mayflower, Arkansas, last March.

It is the largest oil leak in North Dakota since hydraulic fracturing unlocked massive oil reserves in the Bakken shale. In August North Dakota produced more than 911,000 barrels of oil per day.

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