BISMARCK, N.D. — Tribes battling the Dakota Access oil pipeline in court are accusing the Army Corps of Engineers of withholding dozens of documents that could bolster their case that the pipeline could unfairly impact them.

Many of the records that attorneys for the four Sioux tribes allege are missing relate to the pipeline’s crossing beneath the Lake Oahe reservoir on the Missouri River in the Dakotas, which the tribes rely on for drinking water, fishing and religious practices. Fears of a spill into the river sparked prolonged protests in 2016 and early 2017 that drew thousands of pipeline opponents from around the world to southern North Dakota.

The Corps, which permitted the $3.8 billion pipeline that began moving North Dakota oil to Illinois in June 2017, “produced a fragmented and incomplete record designed to defend a flawed agency action, one that omits key documents important to the tribes’ legal challenge,” attorneys for the Standing Rock, Cheyenne River, Yankton and Oglala Sioux tribes wrote in a Wednesday court filing. They implored U.S. District Judge James Boasberg to order the Corps to turn over the requested documents.

The Justice Department, which represents the Corps, declined comment.

Boasberg in June 2017 ruled that the Corps “largely complied” with environmental law when permitting the pipeline built by Texas-based Energy Transfer Partners, but he ordered more study on tribal impacts. The Corps in August 2018 said it had finished more than a year of additional study and that the work substantiated its earlier determination that the pipeline does not pose a higher risk of adverse impacts to minorities.

The tribes are challenging the assertion, hoping to persuade Boasberg to shut down the pipeline.

The Corps in early February turned over to the tribes documents it used in making the determination. The tribes allege records are missing that affect not only their case but also “masks” information that could influence Boasberg’s eventual decision on whether the Corps study is adequate.