WASHINGTON -- In an about-face on the day President Barack Obama announced his re-election bid, Attorney General Eric Holder on Monday ordered that confessed 9/11 mastermind Khalid Sheik Mohammed and four other alleged co-plotters stand trial before a military commission at Guantánamo Bay, Cuba, rather than in a civilian court.

It was an embarrassing reversal nearly a year and a half after Holder announced with much fanfare that the five men, who had been held for years in secret CIA custody before their transfer to the Guantánamo military prison in 2006, would be tried in a courtroom in lower Manhattan.

Holder blamed the decision on Congress for prohibiting the transfer of Guantánamo detainees to the U.S., even for trial.

“We must face a simple truth; those restrictions are unlikely to be repealed in the immediate future. And we simply cannot allow a trial to be delayed any longer for the victims of the 9/11attacks or for their family members who have waited nearly a decade for justice,” Holder said.

The Sept. 11, 2001, attacks killed 2,976 people when four hijacked airliners crashed into the World Trade Center, the Pentagon and a field in southwest Pennsylvania.

Holder also revealed that a grand jury secretly indicted the five men on Dec. 14, 2009, alleging, among other things, that Mohammed had trained the hijackers “by killing sheep and camels” with short-bladed knives similar to the box cutters used in the 9/11 attacks.

That indictment, which was withdrawn Monday, came a year before Congress stepped in and blocked transfers of detainees to U.S. soil. But even before Congress acted, Holder said the Justice Department had delayed the transfer out of concern about security for a trial in Manhattan. At one time, he said, he considered staging the trial at the Otisville Federal Prison, 70 miles northwest of New York, near the Pennsylvania and New Jersey borders.

“The best venue for prosecution was in federal court,” he said. “I stand by that decision today.”

Families who lost relatives in the attacks offered mixed reviews of the decision. Most members of Congress endorsed it.

At the Pentagon, the chief war crimes prosecutor, Navy Capt. John F. Murphy, said his lawyers would prepare charge sheets “in the near future” against Mohammed, 45, Ramzi bin al-Shibh, 37, Walid bin Attash, 32, Ammar al-Baluchi, 32, and Mustafa Hawsawi, 41.

They have been held at Guantánamo since September 2006. The CIA subjected Mohammed to an interrogation technique called waterboarding 183 times. The five were charged in the military commission system during the George W. Bush presidency, but that case was dismissed in November 2009 when Holder announced plans for a civilian trial.

Murphy declined to say whether he would seek the death penalty — a key issue. Mohammed has said previously that he would confess to the plot and seek the death penalty as a fast track to martyrdom. But it is unclear whether a military judge who accepts a guilty plea can also sentence someone to death.

The timing of the announcement was surprising: the day President Barack Obama launched his campaign for re-election. It also came the day before the House Judiciary Committee was to hold a hearing on military commissions, at which relatives of 9/11 victims were expected to hold up pictures of their dead loved ones to protest administration policy.