This story has been updated.

A landmark climate change lawsuit, brought against the federal government by 21 children, has encountered yet another hurdle on its way to trial. A higher court has just stepped in and ordered a temporary stay on the proceedings while it considers an unusual petition from the Trump administration that could prevent from the case from moving forward at all.

The petition, filed last month by the Justice Department with the U.S. Court of Appeals for the 9th Circuit, requests a rarely invoked legal procedure known as a writ of mandamus, which allows higher courts to independently review — and potentially overturn — decisions made by lower courts before they have even held a trial. In this case, the petition calls for the appeals court to step in and independently review a decision made by a federal judge last year to allow the climate lawsuit to move to trial. The Trump administration has also requested a stay on the lawsuit’s proceedings until the 9th Circuit makes a decision on its petition.

On Tuesday, the Court of Appeals for the 9th Circuit ordered a temporary stay until it can make a decision on whether to consider the Trump administration’s requests. According to Julia Olson, chief counsel for the plaintiffs and director of advocacy organization Our Children’s Trust, the 9th Circuit has not actually honored any of the Justice Department’s motions yet. The stay, she said, was enacted “on the court’s own initiative,” and the order notes that “the petition for a writ of mandamus and all other pending motions will be addressed by separate order.”

The Justice Department declined to comment on the new development in the case.

The order comes just three weeks after U.S. Magistrate Judge Thomas Coffin set an official trial date of Feb. 5 for the case. Should the lawsuit get to that point, the plaintiffs — 21 youths now between the ages of 10 and 21 — will argue that the federal government has violated their constitutional rights by promoting the production of greenhouse gases through the use of fossil fuels and damaging the climate system. The case was initially filed during the Obama administration and has since been inherited by President Trump.

But the federal government has been steadily working to stop the case from progressing. The petition for a writ of mandamus is the latest of several measures taken by the Justice Department, including a motion under the Obama administration to have the case dismissed, denied by a federal judge in November, and a subsequent motion under the Trump administration to have this ruling overturned, also denied.

The petition for writ of mandamus, widely considered the “Hail Mary” of legal procedures, is the last measure that could potentially stop the case from going to trial. The request is an “extraordinary” move, according to James May, a law professor and chief sustainability officer at Widener University who is not involved with the lawsuit, and even considering it would be an unusual decision on the part of the 9th Circuit.

In fact, ordering a stay on the lawsuit’s proceedings is a rare move in and of itself, he said. Courts of appeal typically don’t get involved in lower courts’ affairs until the proceedings have progressed to trial, or at least “to a point where there’s something to review,” he told The Washington Post. While he noted that it’s impossible to predict what the Court of Appeals for the 9th Circuit will decide to do next, he added that he doesn’t feel the order is a “positive development” for the plaintiffs.

But Olson remains optimistic.

I don’t think it’s a setback at all,” she said. “And all it means is that they are preparing an order on the Department of Justice’s request for the court to take up this writ of mandamus and motion for a stay. The court has a duty to consider these motions and decide them, and that’s what it’s doing right now.”

And May added that “given the novelty and importance of the claims, it’s possible — but unlikely — that the 9th Circuit needs more time and wants the lower court to take a breath and not do anything that is irreversible in the meantime.”