Fearing contamination of its drinking water, the Oglala Sioux Tribe has entered the legal fray against the Dakota Access pipeline.

Attorneys working for the tribe filed a lawsuit Monday in U.S District Court in Washington calling for the Army Corps of Engineers to halt construction of the controversial oil pipeline until an environmental impact statement is completed.

“After the Corps granted the easement for the DAPL to cross under Lake Oahe and stopped the environmental impact statement process, we really had no choice but to go to court to protect our rights under the 1851 and 1868 Fort Laramie Treaties to safe drinking water,” tribal President Troy “Scott” Weston said in a statement.

The Oglala Sioux filed their complaint the same day that a federal judge in Washington, D.C., denied a similar request from the Cheyenne River Sioux Tribe.

Situated on the Pine Ridge Indian Reservation in southwest South Dakota, the Oglala Sioux Tribe draws its drinking water from the Missouri River, as do more than 200,000 Native and non-Native American people in the state.

“The waters of the Missouri River are sacred to the tribe,” the Oglala Sioux’s suit states. “These waters give life to all of the creatures and plants on the tribe’s lands. The tribe has treaty- and statute-protected property rights to the waters of the Missouri River.”

The tribe gets its water from the Mni Wiconi Rural Water Supply System, which draws water from the Missouri River. Owned and operated by the tribe, the Mni Wiconi rural water system is one of the largest in North America, measuring more than 4,000 miles and serving about 55,000 people in western South Dakota, according to the U.S. Bureau of Reclamation.

Starting from an intake at the Oahe Reservoir near Fort Pierre, the system stretches 150 miles to Pine Ridge, where it provides drinking water for an estimated 21,500 tribal members. The Mni Wiconi water system also supplies water to the Rosebud Sioux and Lower Brule Indian reservations, as well as non-Native communities including Fort Pierre, Murdo, Kadoka, Wall, Philip and Midland.

In 2015, the system supplied those communities with a total of about 78.4 million gallons of water.

On Dec. 4, under the Obama administration, the Army Corps ordered a complete environmental impact statement on the pipeline project. But when the Trump administration took over, the Corps reversed its decision and granted the final easement to allow the completion of pipeline construction near the site of a massive protest movement outside the Standing Rock Sioux Indian Reservation in North Dakota.

The Oglala Sioux lawsuit states that the Army Corps never consulted the tribe about how an oil leak or spill from the Dakota Access pipeline might impact the water from the Mni Wiconi rural water system, the construction of which was completed last September at a federal investment of more than $450 million.