North Dakota Health Department via AP A vacuum truck cleans up oil Tuesday near Tioga, N.D., where more than 20,000 barrels of crude oil have spewed out of a pipeline.

Initial investigations following a 20,600-barrel leak on a Tesoro Logistics pipeline in North Dakota point to corrosion on the 20-year-old pipeline, state regulators said Friday.

The 6-inch pipeline was carrying crude oil from the Bakken shale play to the Stampede rail facility outside Columbus, N.D., when a farmer discovered oil spouting from it Sept. 29.

It is the state's largest oil spill since it became a major U.S. producer. It is also the biggest oil leak on U.S. land since March, when an Exxon Mobil pipeline spilled 5,000 to 7,000 barrels of heavy Canadian crude in Mayflower, Ark.

The release didn't pose an immediate threat to groundwater sources or nearby rivers and lakes, the state Department of Health said Thursday.

Initial concerns that the pipeline was punctured by nearby residents were dismissed, and the cause appears to be corrosion, according to Brian Kalk, chairman of the state Public Service Commission.

"It started out as a small hole and got bigger," Kalk said.

The U.S. Transportation Department's Pipeline and Hazardous Materials Safety Administration oversees the pipeline's operation and is in charge of the investigation.

But the state commission approves the construction of pipelines and operators' compliance with initial designs submitted to the state, Kalk said.

San Antonio, Texas-based Tesoro Logistics declined to comment on what was behind the spill.

"The cause of the release is currently under investigation," said Tina Barbee, a Tesoro spokeswoman.

PHMSA couldn't immediately be reached for comment because of the government shutdown. It wasn't immediately clear how furloughs related to the standoff in Washington could affect the investigation or the restart of the line.

The pipeline, which runs 35 miles from Tioga to Black Slough in North Dakota, was built by BP in 1993.

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

Tesoro bought the pipeline and the refinery from BP in 2001.

Farmer Steven Jensen said Thursday the smell of sweet light crude oil wafted on his farm for four days before he discovered the leak, leading to questions about why the spill wasn't detected sooner.

"These companies, they've got to step up to the plate and use better technology. There is no reason this shouldn't have come up somewhere," Jensen said.

Kalk, of the state Public Service Commission, said he expected the federal investigation to answer questions on why nearby pumping stations failed to dictate a drop in pressure early on and whether maintenance records failed to reveal problems on the line.

"We'd like to know if this could have been prevented or fixed ahead of time," he said.

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