Activists who have demonstrated for months against the Dakota Access Pipeline may have some fuel to justify their protests.

A spill has occurred 150 miles from Cannon Ball, North Dakota, where protesters have fought construction of the Dakota Access Pipeline.

State officials estimate 4,200 barrels of crude oil, or 176,000 gallons, have leaked from the Belle Fourche Pipeline in Billings County.

Of that amount, 130,000 gallons of oil has flowed into Ash Coulee Creek, while the rest leaked onto a hillside, said Bill Suess, spill investigation program manager at the North Dakota Department of Health. Built in the 1980s, the pipeline is 6 inches in diameter and transports about 1,000 barrels of oil daily, he said. The leak happened December 5.

“Any time it gets into water, we respond differently and we take it more seriously,” Suess said. He said more than 100 people are working to clean up the spill. Investigators are still trying to determine the cause, he said.

The incident happened less than a three-hour drive from Cannon Ball, where protests led by the Standing Rock Sioux tribe have simmered for months over the 1,172-mile Dakota Access Pipeline. The $3.7 billion project would connect oil-rich areas of North Dakota to Illinois, where the crude oil could then be transported to refineries on the Gulf Coast or the East Coast.

The demonstrations have turned violent at times.

The Standing Rock Sioux tribe sued the US Army Corps of Engineers after the pipeline was granted final permits in July. The tribe said the project will not only threaten its environmental and economic well-being, but will also cut through sacred land. It said construction would destroy burial sites, prayer sites and culturally significant artifacts.

In early December, the US Army Corps of Engineers announced it would look for an alternate route for the pipeline, although the pipeline is nearly complete.

Companies behind the project have pushed back.

Energy Transfer Partners and Sunoco Logistics Partners said they expect to complete the construction without additional rerouting. They have taken legal action, asking a federal court to allow them to complete the pipeline.

The Dakota Access Pipeline would transport 470,000 barrels of oil a day across four states, Energy Access Partners said.

It will pass through an oil-rich area in North Dakota with an estimated 7.4 billion barrels of undiscovered oil. This oil would be shipped to markets and refineries in the Midwest, East Coast and Gulf Coast regions.

This way, the project developer said, the United States could tap its own backyard for oil, rather than relying on imports from unstable regions of the world.