The lawsuit, filed in October, accuses the Army Corps of illegally allowing the Dakota Access Pipeline to operate without an easement, an environmental impact study or robust emergency spill response plans. The tribe wants a federal court to order that the pipeline be shut down.

Dakota Access LLC asked to join the lawsuit to protect its private business interests in the pipeline’s  operation. Dakota Access spent billions developing the pipeline and has standing contracts requiring DAPL to continue transporting oil, the company wrote in a brief filed in court.

Since 2017, the more than 1,000-mile pipeline — also known as DAPL —  has safely transported more than 1.4 billion barrels of oil, the company wrote.

“Dakota Access remains willing to consider other mitigation measures that the Corps or this Court believe would be appropriate,” the company said.

U.S. District Court Judge James Boasberg approved Dakota Access’ request to join the suit.

In the brief, the company also accuses the Standing Rock Sioux Tribe’s legal challenge of “rehashing old ground.” The tribe previously sued the Army Corps over the pipeline in 2016, seeking to stop DAPL before it finished construction.

That case was also assigned to Boasberg. Boasberg in a 2020 concluded that the Army Corps had erred in granting the pipeline an easement to pass underneath Lake Oahe without a full environmental impact study, as required by the National Environmental Policy Act. He revoked the easement and instructed the Army Corps of Engineers to conduct the study, which is still in the works.

Boasberg ordered the pipeline to be drained of oil until the environmental review was completed, though that demand was ultimately overturned by an appellate court.

The higher court found that shutting down the pipeline was not warranted because the tribe at the time did not have enough evidence that DAPL posed an immediate threat of irreparable harm.

In its latest suit, the Standing Rock Sioux Tribe seeks to present new evidence raising questions about the pipeline’s construction under Lake Oahe.

Standing Rock has long opposed the pipeline, saying it violates the tribe’s sovereignty, has damaged sacred cultural sites and poses a pollution threat to the tribe’s water supply.

The Army Corps of Engineers regulates a section of the pipeline that passes underneath Lake Oahe, a reservoir on the Missouri River, less than a half-mile upstream from the Standing Rock Reservation.

“The Corps has failed to act and failed to protect the tribe,” Standing Rock Chairwoman Janet Alkire said in an October press conference announcing the lawsuit.

The pipeline carries oil from northwest North Dakota to southwest Illinois. Its path includes unceded land recognized as belonging to the Sioux Nation under an 1851 treaty with the U.S. government.

North Dakota and 13 other states have also intervened in the case on the side of the Army Corps of Engineers.

The 13 states that joined the lawsuit are Iowa, Georgia, Indiana, Kentucky, Louisiana, Missouri, Montana, Nebraska, Oklahoma, South Carolina, South Dakota, Texas and West Virginia.

The states have argued shutting down DAPL would disrupt the regional economy, violate state sovereignty, and make road and rail transit more dangerous.

The pipeline has provided tens of millions of dollars in tax revenue to North Dakota, South Dakota, Iowa and Illinois.