The Dakota Access oil pipeline’s operators plan to ask the U.S. Supreme Court to intervene in its ongoing legal battle to keep the line out of North Dakota open, according to a court filing on April 29.

The operators of the 570,000 bbl/d line, led by Energy Transfer LP, sought a stay from the U.S. Court of Appeals for the District of Columbia, saying it would allow the Dakota Access Pipeline (DAPL) to continue running.

The U.S. District Court for the District of Columbia earlier this week ordered the U.S. Army Corps of Engineers to provide an update by May 3 on when it plans to complete an environmental review of the pipeline and whether it recommends the line should shut during the process.

DAPL’s operators said they do not intend to reargue whether an environmental assessment is required but said that they could show reasonable probability that the Supreme Court would reverse the lower courts’ decision to scrap a key environmental permit for the line.

It remains uncertain whether the case would be taken up by the highest U.S. court, which only hears about 2% of the cases brought to it for review each year.

“They are working very hard to avoid the environmental review,” said Earthjustice attorney Jan Hasselman, who represents the Standing Rock Sioux Tribe. “That’s all that the court has ordered so far is a full and careful look at the risks and impact of this pipeline and their furious opposition speaks volumes,” he said.

DAPL, the biggest pipeline out of the Bakken shale basin, began operating in mid-2017 after drawing controversy during construction as Native American tribes and activists protested its route under the Dakotas’ Lake Oahe, a critical drinking water source for the tribes.

The pipeline has continued to operate despite losing its easement to run beneath the lake in July, when the district court threw out its permit and ordered the new environmental assessment. That court is expected to rule in the next several weeks on whether it will order DAPL to shut pending the outcome of the review.