This story was updated at 2.00 a.m. ET on February 16, 2017 to reflect news events.

Native American tribes have filed yet another legal motion seeking to halt construction of the Dakota Access pipeline. Legal experts said the attempt faces long odds but may be the tribes’ best hope for blocking the project.

The motion, filed Tuesday by the Standing Rock and Cheyenne River Sioux tribes, asks the court to reverse an easement for the pipeline that the Army Corps of Engineers granted. That easement lifted the final hurdle for the project’s completion.

The tribes said the Corps’ actions violate the National Environmental Policy Act and the Corps’ responsibility to protect the tribes’ treaty rights. They called the decision “arbitrary, capricious, and contrary to law.”

The move is the latest in a long and twisting legal battle and protest movement to block the project’s Missouri River crossing under Lake Oahe, which skirts the Standing Rock Sioux reservation. The tribe says the pipeline threatens its water supply and sacred sites. In the final weeks of the Obama administration, the Army Corps declined to issue the final easement for the pipeline, saying it would conduct an environmental impact statement and would consider rerouting the pipeline.

One of Donald Trump’s first actions as president, however, was to order a reversal of the decision and swift approval of the project. Last week, the Corps issued the easement for the Lake Oahe crossing without producing an impact statement. The pipeline now may be just weeks away from completion.

The tribes have had little success in their legal attempts to block construction, said Patrick A. Parenteau, a professor at Vermont Law School.

“It’s already knocked them back a few times. It’s never a good sign when you keep losing,” he said. But Parenteau said the motion contains the best arguments the tribes have put forward.

“The strongest possible argument is that the Trump administration, with no change in facts, no change in conditions, reversed the government’s position,” he said. “As the federal government, you told this court in December you had a legal obligation to do a full environmental impact statement. Now you’re telling me you don’t. Why?”

Courts generally grant agencies substantial deference on how to proceed under the environmental policy act, said Wayne J. D’Angelo, an energy and environmental lawyer with Kelley, Drye & Warren, which makes the tribes’ argument unlikely to succeed. “They’re throwing everything against the wall,” he said. “The filing reflects the last of the plaintiffs’ actions for court intervention here.”

The tribes’ best hope may be that the Corps gave no substantive explanation for why it reversed itself, said Keith Benes, a former State Department lawyer who’s now a consultant and a fellow at Columbia University’s Center on Global Energy Policy. “Courts say an agency can change their mind about something, but courts also say that an agency has to give a reason,” he said. “By doing what it did, the Army really strengthened the case that the tribes had.”

On Monday, a federal district judge denied a separate attempt by the Cheyenne River Sioux to grant a temporary restraining order after the tribe argued the pipeline could pollute water that it uses in religious ceremonies. While the judge decided not to halt construction, that case will proceed and the judge said he would rule on the merits of the challenge before oil begins to flow.

The pipeline, which is being built by Energy Transfer Partners, has triggered ongoing protests led by tribal groups. While most demonstrators have left the site of the main protest encampment in North Dakota, a few hundred remain. Earlier this month, the Corps told tribal leaders that it would begin clearing the protest site because of safety concerns and the risk of floods. This week, as warm temperatures have prompted rapid snowmelt, state officials warned that flooding could contaminate nearby rivers with garbage and waste that protesters have left behind. On Wednesday, North Dakota Gov. Doug Burgum ordered protesters to vacate the federal land by Feb. 22.

The easement being protested would cover a short section of the pipeline, most of which has already been constructed. This week’s filing represents one of the tribe’s last options for blocking the project. A ruling is expected quickly, but could be appealed.

D’Angelo said that while he expects the judge to deny the motion, there’s little legal precedent for how and when a new administration can shift course on a decision like this.

“There’s not a ton of case law out there on it,” he said. “I think how courts view these policy shifts back and forth is interesting, and this is an important issue.”