WASHINGTON >> The Biden administration asked a federal appeals court on Tuesday to block a plea agreement for accused 9/11 mastermind Khalid Sheikh Mohammed that would spare him the risk of the death penalty in one of the deadliest attacks ever on the United States.

The Justice Department argued in a brief filed with a federal appeals court in the District of Columbia that the government would be irreparably harmed if the guilty pleas were accepted for Mohammed and two co-defendants in the Sept. 11, 2001, terror attacks that killed some 3,000 people in New York, Pennsylvania and Washington, D.C.

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.”

The Defense Department negotiated and approved the plea deal but later repudiated it. Attorneys for the defendants argue the deal is already legally in effect and that Defense Secretary Lloyd Austin, who began the administration’s efforts to throw it out, acted too late.

Families react

Family members have been split on the deal, with some calling it the best resolution possible for a prosecution mired for more than a decade in pretrial hearings and legal and logistical difficulties. Others demanded a trial and — they hoped — execution.

Some legal experts have warned that the legal challenges posed by the case, including the men’s torture under CIA custody after their capture, could keep the aging detainees from ever facing verdicts and any possible sentences.

Military prosecutors this summer notified families of the victims that the senior Pentagon official overseeing Guantanamo had approved a plea deal after more than two years of negotiations. The deal was “the best path to finality and justice,” military prosecutors said.

But some family members and Republican lawmakers condemned the deal and the Biden administration for reaching it.

Austin has fought unsuccessfully since August to throw out the agreement, saying that a decision on death penalties in an attack as grave as the Sept. 11 plot should only be made by the defense secretary.

A military judge at Guantanamo and a military appeals panel rejected those efforts, saying he had no power to throw out the agreement after it had been approved by the senior Pentagon official for Guantanamo.