A federal judge ruled Wednesday that the environmental review for the Dakota Access pipeline was, in part, inadequate and must be reconsidered, handing tribal opponents of the 1,170-mile pipeline project a key legal victory.

But U.S. District Judge James Boasberg did not order pipeline operators to stop the oil that is already flowing through the project, saying he would need to consider that request in light of Wednesday's judgement.

Boasberg ruled that the federal government “substantially complied” with the federal environmental permitting law that governs projects such as Dakota Access, a $3.8 billion pipeline that can carry up to 570,000 barrels of oil per day.

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But, Boasberg wrote in a 91-page opinion, the Army Corps of Engineers “did not adequately consider the impacts of an oil spill on fishing rights, hunting rights, or environmental justice, or the degree to which the pipeline’s effects are likely to be highly controversial.”

He ruled that the Army Corps, which permitted the project, would need to conduct a new review of Dakota Access that considers those factors.

But Boasberg did not order Dakota Access to cease operations, which have been underway since June 1. He said that is a “separate question” that he will consider in the future.

Two tribes, the Standing Rock Sioux and Cheyenne River Sioux, have tried for months to halt the Dakota Access project. They argue the pipeline, which crosses the Missouri River upstream from the Standing Rock Indian Reservation in North Dakota, threatens water quality there.

Courts have rejected two previous arguments against the pipeline: that it violates the tribes’ religious liberty by flowing under sacred water in North Dakota and that its construction threatened cultural heritage sites on the Great Plains.

The tribes have long argued the project needs to undergo a more thorough environmental review before it could transport crude from the Bakken oil fields to terminals in Illinois.

The Army Corps said last summer that it would grant the permits necessary to build the project, but opponents said the pipeline needed to go through a more rigorous Environmental Impact Statement (EIS) review first.

The Obama administration agreed. Last year, officials refused to issue a construction permit for the segment of the project that runs along federal land, opting instead to kick off a potentially lengthy EIS review.

President Trump in January reversed that decision, signing a presidential order to build the pipeline. Regulators issued construction permits in February and oil began flowing through the completed pipeline earlier this month.

Boasberg ruled that part of the original permitting decision that cleared the way for the pipeline was inadequate, as the tribes had insisted.

“The agency failed to adequately consider the impacts of an oil spill on Standing Rock’s fishing and hunting rights and on environmental justice, and in February 2017, it did not sufficiently weigh the degree to which the project’s effects are likely to be highly controversial in light of critiques of its scientific methods and data,” he wrote.

But Boasberg defended many of the federal decisions that went into permuting the project, saying the Army Corps “substantially complied with [the National Environmental Policy Act] in many areas.”

He also rejected the tribes’ argument that Trump’s decision that allowed the project to move forward was illegal.

Boasberg has scheduled a briefing next week to determine what happens next, including hearing tribal arguments that oil should stop running through the pipeline while regulators reconsider the project’s permits.

Standing Rock Sioux Chairman Dave Archambault hailed the decision as a “major victory” in a statement Wednesday.

“We applaud the courts for protecting our laws and regulations from undue political influence, and will ask the Court to shut down pipeline operations immediately,” he said.