One of two American Indian tribes fighting the Dakota Access oil pipeline has filed a legal challenge to try to block its completion.

The Cheyenne River Sioux worries a pipeline leak could contaminate its drinking water.

The tribe filed a legal challenge in federal court in Washington, D.C., on Thursday. The Army on Wednesday gave Energy Transfer Partners formal permission to lay pipe under a Missouri River reservoir in North Dakota. That’s the last big chunk of construction for the $3.8 billion pipeline to carry North Dakota oil to Illinois. The work is underway.

The Dallas-based pipeline developer says it will be safe.

President Donald Trump signed an executive action in January instructing the Army Corps of Engineers to advance pipeline construction. The tribes argue that violates treaty rights.

The Standing Rock Sioux tribe has also vowed to fight the construction in court.

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