WASHINGTON (AP) The Biden administration succeeded Thursday in temporarily blocking a guilty plea by accused 9/11 mastermind Khalid Sheikh Mohammed, stalling a plea deal that would spare him the risk of the death penalty for al-Qaida’s Sept. 11, 2001, attacks on the United States.

Administration lawyers had urged a federal appeals panel to block Mohammed’s guilty plea from going forward as scheduled Friday at Guantanamo Bay, Cuba.

Defense lawyers described the attempts to throw out the agreement as the latest in two decades of “fitful” and “negligent” mishandling of the case by the U.S. military and successive administrations.

The federal appeals panel agreed Thursday evening to a temporary stay. It stressed that the stay would hold only as long as it took to more fully consider arguments in the government’s request, and should not be considered a final ruling.

It stalls an attempt to wrap up more than two decades of military prosecution beset by legal challenges in one of the deadliest attacks in U.S. history.

Thursday’s legal action marked a last-ditch government request to prevent Mohammed from entering a guilty plea Friday. Family members of some of the nearly 3,000 people killed in al-Qaida’s Sept. 11, 2001, attacks already were gathered at the U.S. naval base in Guantanamo Bay, Cuba, to hear it.

The fight has put the Biden administration at odds with the U.S. military officials it had appointed to oversee justice in al-Qaida’s attacks on Sept. 11, 2001, that killed nearly 3,000 people. It was the latest tumult and uncertainty in two decades of troubled prosecution tied to one of the deadliest attacks on American soil.

Asked about the appeal Thursday after a meeting in Germany with allies about military support for Ukraine, Defense Secretary Lloyd Austin told reporters he had not changed his opposition to the deal and cited the court challenge in declining further comment.

The deal, negotiated over two years and approved by military prosecutors and the Pentagon’s senior official for Guantanamo in late July, would spare Mohammed and two co-defendants the risk of the death penalty. It also obligates them to answer any lingering questions that families of the victims have about the attacks.

Defense attorneys say the plea agreements are already in effect and say Austin has no legal authority to throw them out after the fact.

At Guantanamo, preparations have moved ahead for Friday’s proceedings, and family members of some of the victims already have gathered. If the hearing goes forward, Mohammed would swear an oath in the military courtroom and then defense attorney Gary Sowards would enter pleas on his behalf to 2,976 counts of murder, along with other charges.

Pleas by co-defendants Walid bin Attash and Mustafa al Hawsawi would follow later this month. Attorneys say months of sentencing hearings to follow would give the government an opportunity to outline its case and allow families to speak of their loss.

Legal and logistical challenges have bogged down the 9/11 case in the 17 years since charges were filed against Mohammed, who prosecutors say conceived the idea of using hijacked planes in the attacks. The case remains in pretrial hearings, with no trial date set.

Years of defense and prosecution testimony have dragged on about how much the torture of Mohammed and other defendants in CIA custody renders their later statements unusable in court.

With that in mind, military prosecutors notified families of the victims this summer that the senior Pentagon official overseeing Guantanamo had approved a plea deal. They called it “the best path to finality and justice.”

Austin unexpectedly announced Aug. 2 that he was scrapping the agreement. He argued that a decision on death penalties in an attack as grave as Sept. 11 should only be made by the defense secretary.

The Biden administration went to the District of Columbia federal appeals court this week after the Guantanamo judge and a military review panel rejected Austin’s intervention.

Mohammed’s attorneys argued that Austin’s “extraordinary intervention in this case is solely a product of his lack of oversight over his own duly appointed delegate,” meaning the senior Pentagon official overseeing Guantanamo.

The Justice Department said the government would be irreparably harmed if the guilty pleas were accepted.

It said the government would be denied a chance for a public trial and the opportunity to “seek capital punishment against three men charged with a heinous act of mass murder that caused the death of thousands of people and shocked the nation and the world.”

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AP reporter Tara Copp contributed from Ramstein Air Base, Germany.