“It is my hope that today's announcement means that long-delayed accountability will finally be served on the terrorists who killed thousands of Americans on 9/11,” said retired Navy Cmdr. Kirk Lippold, who was the captain of the USS Cole in October 2000 when an al-Qaida suicide bomber attacked the destroyer off Yemen, killing 17 U.S. sailors.

Lippold, an architect of Bush-era Guantánamo policy, accused the Obama administration of “political waffling” rather than “fortitude and leadership.”

The September 11th Families for Peaceful Tomorrows group said it was “profoundly disappointed” by “a step backward in our hopes that justice will be served.”

“The shame of Guantánamo will continue,” the group said in a prepared statement. “As families of those who were murdered on that day, we have waited nearly 10 years to see those who committed these savage criminal acts to be brought to justice.”

Reaction from Congress, which had voted to block a U.S. federal trial through funding bans, ranged from triumph to relief that New York would be spared the trial.

“As I have been saying all along, these terror trials belong in a military commission at Guantánamo. I am absolutely shocked that it took Attorney General Holder 507 days to come to this realization,” said Rep. Peter King, R-N.Y., chairman of the House Homeland Security Committee.

Sen. Lindsey Graham, R-S.C., who helped create the military commission system, said he appreciated Holder's decision to use it for the trials.

“Military commissions have been used in wars throughout our history, and they should be used in this war,” said Graham, a military lawyer who served in Iraq and Afghanistan. “The commission trials will be transparent, conducted by the same judges and jurors who administer justice to our own troops, and subject to civilian review. In addition, the military commissions system balances the interest of the accused with the safety of our nation as a whole in this time of war.”

Sen. Patrick Leahy, D-Vt., chairman of the Senate Judiciary Committee, lamented the reversal. He called the American justice system “the envy of the world” and with hundreds of terrorism convictions already “more than capable of trying high-profile terrorism and national security cases.”

“The record in military commissions pales in comparison, with only a handful of convictions, and the ground rules still in flux,” he said.

The decision resurrects a long-standing dispute over whether a military commission would be as fair as a civilian trial and whether its verdict would be as respected.

The National Association of Criminal Defense Lawyers worried that the military wouldn't provide adequate funding to hire civilian defense lawyers with death-penalty experience. The group also said the Obama administration hadn't gone far enough to ensure that the evidence used in a case would meet civilian court standards.

“Despite some cosmetic changes since the Bush-era commissions, the commission rules still permit the government to introduce secret evidence, hearsay and statements obtained through coercion,” said Norman Reimer, the group's executive director.

As a candidate and senator, Obama condemned the commissions. As president, he worked with Congress to reform them.

“It's devastating to the rule of law,” said Anthony Romero, executive director of the American Civil Liberties Union, blaming “politics” for the decision and arguing that the military commissions aren't capable of keeping out evidence gleaned from CIA interrogations, often under harsh conditions.

“The very Defense Department that enabled the torture is now going to be the adjudicator of justice,” he said.

Dean Boyd of the Justice Department countered that under Obama, “among the key reforms to the military commissions was a ban on the admissibility of statements obtained by the use of cruel, inhuman or degrading treatment.”

Holder also endorsed the commissions. “I believe they can deliver fair trials and just verdicts,” he said.

Rosenberg reports for the Miami Herald. James Rosen, Lesley Clark, Greg Gordon, Margaret Talev and Marisa Taylor contributed to this report.