President Obama signed an executive order Monday that will create a formal system of indefinite detention for those held at the U.S. military prison at Guantanamo Bay, Cuba, who continue to pose a significant threat to national security. The administration also said it will start new military commission trials for detainees there.

The announcements, coming more than two years after Obama vowed in another executive order to close the detention center, all but cements Guantanamo Bay's continuing role in U.S. counterterrorism policy.

Administration officials said the president is still committed to closing the prison, although he made no mention of that goal in a short statement Monday. The administration's original plans to create a detention center in the United States and prosecute some detainees in federal court have all but collapsed in the face of bipartisan congressional opposition.

The executive order recognizes the reality that some Guantanamo Bay detainees will remain in U.S. custody for many years, if not for life. The new system allows them the prospect of successfully arguing in the future that they should be released because they do not pose a threat.

"Today, I am announcing several steps that broaden our ability to bring terrorists to justice, provide oversight for our actions and ensure the humane treatment of detainees," Obama said in statement. "I strongly believe that the American system of justice is a key part of our arsenal in the war against al-Qaeda and its affiliates, and we will continue to draw on all aspects of our justice system - including [federal] Article III Courts - to ensure that our security and our values are strengthened."

But activists on either end of the debate over closing the prison cast the announcement as a reversal.

"It is virtually impossible to imagine how one closes Guantanamo in light of this executive order," said Anthony Romero, executive director of the American Civil Liberties Union. "In a little over two years, the Obama administration has done a complete about-face."

Rep. Peter T. King (R-N.Y.), chairman of the House Homeland Security Committee, said the order vindicated Obama's predecessor. "I commend the Obama Administration for issuing this Executive Order," he said in a statement. "The bottom line is that it affirms the Bush Administration policy that our government has the right to detain dangerous terrorists until the cessation of hostilities."

The executive order applies to at least 48 of the 172 detainees who remain at Guantanamo Bay. An inter-agency panel led by Justice Department lawyers determined that this group could not be prosecuted in military commissions or in federal court because evidentiary problems would hamper a trial. But intelligence assessments also concluded that these detainees remain a serious threat and could not be safely repatriated or resettled in a third country. The administration said it will hold reviews for detainees it plans to prosecute but has not charged.

The administration argues that it has the legal authority to continue to hold all of the detainees at Guantanamo Bay under the laws of war. Federal courts have backed that assertion, although they have found that some detainees should be released for a lack of evidence against them. The detainees will continue to have the right to petition the federal courts under the doctrine of habeas corpus.

"The new executive order doesn't change the legal authority for detention at all," said Kate Martin, director of the Center for National Security Studies. "It simply provides additional reviews for individuals who have been found by the habeas courts to be lawfully detained under the laws of war."

Under the order, each detainee will receive within a year a written, unclassified review of the factors that justify his continued incarceration. A government representative will be appointed to advocate on behalf of the detainees, who will have the right to hire private counsel, but not at the government's expense, the executive order says.