U.S. District Judge James Boasberg ruled Wednesday that the Army Corps of Engineers didn’t adequately consider the possible impacts of an oil spill where the pipeline passes under Lake Oahe, a wide area of the Missouri River, in North Dakota.

Boasberg says the Army must redo its environmental analysis in certain sections and he’ll consider later whether the pipeline must halt operations in the meantime.

The Standing Rock Sioux tribe, which has led opposition to the pipeline, called it a “significant victory.”

A spokeswoman for Texas-based Energy Transfer Partners, the pipeline’s developer, didn’t immediately respond to messages.