"This wasn't a decision that I took lightly," Austin told reporters in Maryland.
"But I have long believed that the families of the victims, our service members, and the American public deserve the opportunity to see military commissions, commission trials carried out" in the 9/11 case, he said.
US Defence Secretary Lloyd Austin says he didn't take the decision lightly. (AP PHOTO)
At the US military commission in Guantanamo Bay, Cuba, meanwhile, lawyers and the judge in the case of accused 9/11 mastermind Khalid Sheikh Mohammed and two other defendants who had also taken plea deals were still coming to terms with Austin's surprise order and its effect on efforts to resolve the more than 20-year-old case.
Mohammed and a total of four other defendants at Guantanamo are accused in the September 11, 2001, attacks, using hijacked passenger jets, that killed nearly 3000 people in New York, Washington and Pennsylvania. A fourth defendant did not agree to the plea agreement, and a fifth defendant last year was ruled mentally unfit to continue facing trial.
Legal complications, including questions over how much the men's torture while in CIA custody in the first years after their capture has tainted the evidence and the case itself, have helped drag out proceedings. The case remains in pre-trial hearings after more than a decade.
After about two years of plea negotiations, the Pentagon-appointed retired general overseeing the military commission last week approved a plea bargain struck by prosecutors and defence lawyers that would have spared Mohammed and the two others the risk of the death penalty, in return for their guilty pleas.
Families of 9/11 victims offered differing opinions, with some welcoming the resolution and others saying they wanted to see capital trials. Senior Republicans in Congress publicly lambasted the Biden administration for the plea bargain.
An order from Austin made public on Friday, in which he said he was revoking approval of the plea bargain and personally assuming that decision-making authority in the 9/11 case, upended the deal.
"There's not a day that goes by when I don't think of 9/11 and the Americans that were murdered that day," he said..
Some of the lawyers and civil-rights organisations involved in the Guantanamo Bay cases accuse Austin of bowing to political pressure in overturning the plea deal, and are challenging the legality of Austin's action.
Open hearings are scheduled to resume on Wednesday for the first time since Austin's order, and may reveal the defence's response to the overriding of the plea bargain.
