BISMARCK, N.D. — The Army told Congress on Tuesday that it will allow the $3.8 billion Dakota Access oil pipeline to cross under a Missouri River reservoir in North Dakota, completing the disputed four-state project.

The Army intends to allow the Lake Oahe crossing as early as Wednesday, according to court documents the Justice Department filed that include letters to members of Congress from Deputy Assistant Army Secretary Paul Cramer.

The stretch under Lake Oahe is the final big chunk of work on the 1,200-mile pipeline that would carry North Dakota oil through the Dakotas and Iowa to a shipping point in Illinois. Developer Energy Transfer Partners had hoped to have oil flowing through the pipeline by the end of 2016, but construction has been stalled while the Army Corps of Engineers and the Dallas-based company battled in court over the crossing.

The Standing Rock Sioux, whose reservation is just downstream from the crossing, fears a leak would pollute its drinking water and is likely to file a legal challenge to the Army’s decision to grant an easement. The tribe has led protests that drew hundreds and at times thousands of people who dubbed themselves “water protectors” to an encampment near the crossing. ETP says the pipeline is safe.

An environmental assessment conducted last year determined the crossing would not have a significant impact on the environment. However, then-Assistant Army Secretary for Civil Works Jo-Ellen Darcy on Dec. 4 declined to issue permission for the crossing, saying a broader environmental study was warranted given the Standing Rock Sioux’s opposition.

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ETP called Darcy’s decision politically motivated and accused then-President Barack Obama’s administration of delaying the matter until he left office. The Corps launched a study of the crossing on Jan. 18, two days before Obama left office, that could have taken up to two years to complete. President Donald Trump signed an executive action on Jan. 24 telling the Corps to quickly reconsider Darcy’s decision.

The court documents filed Tuesday include a proposed Federal Register notice terminating the study.

ETP has been poised to begin drilling under the lake as soon as it has approval. Workers have drilled entry and exit holes for the Oahe crossing, and oil has been put in the pipeline leading up to the lake in anticipation of finishing the project.

Protesters have at times clashed with police, leading to nearly 700 arrests since the tribe set up the encampment on federal land. The camp’s population thinned to fewer than 300 as harsh winter weather arrived and as Standing Rock officials pleaded for the camp to disband before the spring flooding season.