A U.S. Air Force judge on Wednesday ordered Khalid Sheikh ‌Mohammed, the accused mastermind of the September 11, 2001, attacks on the United States, and three co-defendants to go on trial before a military tribunal in June 2028.

The trial-scheduling order entered by Air Force Lieutenant Colonel Michael Schrama marked the latest ​milestone in a military prosecution beset by two decades of legal deadlock surrounding the four detainees ​held in Guantanamo Bay, Cuba.

Last month, a U.S. appeals court refused to allow Mohammed ⁠and two of his co-defendants to plead guilty under agreements that would have spared them the death penalty.

The appellate decision reversed the decisions of a military judge and the U.S. Court of Military Commission Review in support of the plea deal.

The agreement had been offered last year and accepted by the official who oversees the Pentagon's Guantanamo war court, only to be revoked last August by then-Defense Secretary Lloyd Austin amid criticism of the deal from Republican lawmakers.

Under Schrama's 11-page ​order, Mohammed and three co-defendants — Walid Muhammad Salih Mubarak bin 'Atash, Mustafa Ahmed Adam al Hawsawi, and Ali ​Abdul Aziz Ali — will stand trial starting on June 5, 2028.

The proceedings open with the selection of a panel consisting entirely of ‌service members who will act as the jury to hear the case according to the rules of military justice.

Opening statements will commence 30 calendar days after the panel is seated, followed by presentation of the prosecution's evidence, after which the defendants may file motions for findings of not guilty.

The order gives both sides opportunities to argue those motions and rebut one another, and for prosecutors to reopen their case. The defence presents its principal evidence starting 60 calendar days after the prosecution concludes its case.

The trial ends with a sentencing phase.

Schrama rejected the prosecutors' proposal to open the trial in January 2027, saying that would leave too little time for the "resolution of pretrial evidentiary and compliance motions."

Mohammed remains the most well-known inmate at the Guantanamo detention facility, which was set up in 2002 by then-U.S. President George W. Bush to house foreign militant suspects following the ⁠September 11, ​2001, attacks.

He is accused of orchestrating the plot to fly hijacked ​passenger jetliners into the World Trade Centre in New York City and into the Pentagon. The 9/11 attacks killed nearly 3,000 people.