GUANTANAMO BAY NAVAL BASE, Cuba - A persistent knock came from inside the heavy, locked cell door.
A young U.S. Army guard strode over and leaned in to hear the detainee through a shatterproof window.
"What do you want?" the guard asked, not unkindly, in one of the many daily moments in which suspected terrorists demand to be dealt with as their lives hang in legal limbo.
During nearly 12 years of legal disputes and political battles, the United States has put off deciding the fate of al-Qaida and Taliban militants who were captured after the Sept. 11 attacks but denied quick or full access to the American justice system.
Now, as Congress considers whether to grant trials and transfers to most detainees, time may be running out on the law that allows the U.S. to hold them.
The 2001 law is known as the Authorization for the Use of Military Force, or AUMF. It allowed the U.S. military to invade Afghanistan to pursue, detain and punish extremists linked to the attacks of Sept. 11, 2001. The law has been used to justify attacks on militants in Pakistan, Yemen, Somalia and elsewhere.
Will it remain valid if U.S. combat troops withdraw from Afghanistan at the end of 2014 - whether thousands stay as trainers or if the U.S. pulls out entirely? That's an open legal question that, officials and experts say, must be resolved over the next year.
"The jury is still out on when the AUMF might expire," said Army Lt. Col Todd Breasseale, a Pentagon spokesman. "Many argue that's not set."
If U.S. troops withdraw, "it certainly increases the pressure, as some administration officials have argued, to decide whether the AUMF should remain in effect as is, or if a new version is necessary," Breasseale said in a statement.
In 2009, on the second day of his presidency, Obama ordered the terrorist detention center at Guantanamo Bay, Cuba, to be closed within one year. Obama long has derided the facility, where critics say detainees have been abused, interrogated and held illegally, as a blow to American values and credibility worldwide.
Opponents in Congress refuse to let the detainees come to the U.S. for trial, citing security risks to Americans. Lawmakers have blocked the transfer and resettlement of most of the remaining detainees to other nations, fearing they will return to terrorist havens upon their release. Nearly 30 percent of Guantanamo detainees who have been released have since resumed the fight. Today, 164 detainees are held at Guantanamo, down from a peak of about 660 a decade ago. Most were tried, transferred or cleared for release under President George W. Bush. Seventy-eight have left since Obama took office.
The sprawling camp of barbed wire and hardened cell blocks costs U.S. taxpayers about $454 million each year; that comes to about $2.7 million per detainee.
The facility shows no signs of shutting down beyond a temporary budget freeze on the detainees' library, where well-worn copies of the Quran, the "Hunger Games" series and Obama's book, "The Audacity of Hope," are among the 6,000 titles available for reading.
New housing is being built for some of the estimated 5,500 U.S. troops and contractors at the Navy base. More than one-third of them work for the detention camp. Medical staff openly discuss how they will care for aging detainees in coming years.
The Republican-led U.S. House has written legislation that requires the Pentagon to give Congress an annual plan for Guantanamo until the youngest detainee, now in his late 20s, turns 66, meaning the detention camp could remain open for nearly 40 more years.