WASHINGTON - Long-awaited US moves to close the controversial detention center for "war on terror" suspects at Guantanamo Bay will run into an array of complex procedural and legal pitfalls, analysts and observers say.

About 245 prisoners are still held at the jail housed at a US naval base in southern Cuba, which has been a rallying point for anti-American sentiment around the world for the last eight years.

President Barack Obama Thursday signed an executive order to close the prison within a year, and set up a review board co-chaired by the Justice Department and the Pentagon, among other agencies, to sift through the evidence in each individual case.

Vice President Joseph Biden Sunday tried to ease American concerns, saying none of the prisoners would be released into the United States, unless they had legal residency.

"We won't release people inside the United States because all but one I believe is not an American citizen, an American national," Biden told CBS's "Face the Nation" on Sunday.

"We're going one prisoner at a time. We're trying to figure out exactly what we inherited," he added, asking people to give the administration time to resolve outstanding questions.

The inmates are likely to fall into three categories. Those who the US says it is holding with no evidence of wrong-doing will be transferred and subsequently released, most probably back to their home countries.

The second group of detainees will be prosecuted according to the evidence held by the government.

The third and most troubling group for the Obama administration are those deemed too dangerous for release, but against whom the government has either not enough credible evidence or only unusable confessions obtained through "enhanced" interrogation -- or torture, as some charge.

For this group the review board may seek legal means to keep them in detention, perhaps indefinitely.

Obama's Guantanamo decree this week lays out "a process and an aspiration," but it carefully "keeps all options on the table and gives the new administration wide latitude to make policy as it learns over the coming year exactly whom it is dealing with," said Brookings Institution fellow Benjamin Wittes.

Civil liberties and human rights associations, along with a number of legal groups, have argued that holding someone indefinitely without charge cannot be unjustified.

But the revelation Friday that two former detainees at Guantanamo have been elevated to the senior ranks of Al-Qaeda is set to heighten debate over what to do with prisoners believed to still pose a danger.

Republican senator John McCain, defeated by Obama in the 2008 elections, said Sunday that while the prison should be closed, the president should first have worked out other issues, including what to do with the prisoners.

"Where are you going to send them? That decision I would have made before I'd announced the closure, because I don't know of a state in America that wants them in their state," McCain told "Fox News Sunday."

"You think Yucca Mountain is a NIMBY problem? Wait till you see this one," he said, referring to a controversial proposed nuclear waste dump in Nevada.

In a further complication, the Supreme Court ruled in June that detainees can challenge their detention in US federal courts in Washington, meaning that different legal reviews of Guantanamo cases may take place in parallel.

The Obama administration has requested a postponement of legal hearings in three cases, but "the detainee's lawyer can either consent to or oppose such a request," said David Cynamon, a lawyer representing four Kuwaiti detainees.

"It would then be up to the court to decide whether to grant a delay in each such case," he added.

One of the unanswerable questions facing detainees is not whether they will appear in court, but whether that court will be a federal one, or a military commission.

In a commission the top secret nature of the evidence held against some detainees creates unique challenges for the US justice system, in some cases trapping the inmates in a legal web akin to a Franz Kafka novel.

Republican lawmaker Lindsay Graham told CNN on Sunday that he believed the military commissions could be reformed, but should still be the ones to try the detainees.

"Enemy combatants need to be held off the battlefield as long as they are a threat. The worse thing we could do is criminalize this war," he said.

"We're not fighting a bunch of criminals, we are fighting warriors committed our destruction and we need to get this right."