Guantanamo Naval Base, Cuba ? This was a sleepy Navy outpost before the U.S. began using it to hold prisoners in the wake of the Sept. 11 attacks – and it may soon become one again.

It is increasingly obvious that the days of this U.S. offshore prison are numbered. The Bush administration’s main rationale for holding terrorism suspects without trial vanished when the Supreme Court ruled on June 12 that they have certain legal rights. John McCain and Barack Obama have both called for the detention center to be shut.

But whoever becomes the new president will have to figure out what to do with those left at Guantanamo – roughly 270 at present.

“It’s pretty easy to say ‘Let’s close Guantanamo,”‘ Navy Rear Adm. Mark Buzby said in an interview before leaving as commander of the detention center last month. “But the fact of the matter is there are some pretty dangerous people that have to be kept someplace.”

McCain, the presumptive Republican nominee for president, has said he wants to move the detainees to the military’s prison at Fort Leavenworth, Kansas. But finding room for them all might be a problem – just over 400 inmates are now locked up at Fort Leavenworth, which has a capacity for 515.

McCain wants the prisoners tried at military commissions, or war crimes courts, which are allowed under a 2006 law that he supported. These commissions act as criminal courts run by the U.S. armed forces to try those considered enemies during wartime. So far, 19 Guantanamo detainees have been charged in such commissions.

Obama, the presumed Democratic nominee, said he would close Guantanamo and move the detainees to both civilian and military facilities in the United States, including Leavenworth, according to campaign spokesman Reid Cherlin. Obama wants the detainees to be tried in federal criminal courts or in military courts martial.

The Pentagon now plans to try about 80 prisoners at military commissions, but another 130 are considered too dangerous to let go and won’t be prosecuted. About 60 are slated for transfer from Guantanamo, but the Pentagon says they can’t go home because their governments won’t accept them, might release them and create a security risk for the U.S., or might even torture them.

Defense Secretary Robert Gates recently told lawmakers he too wants Guantanamo’s prison shut down, but added: “We’re stuck in several ways.”

In general, convictions would be harder to secure in federal courts, but would also stand up better in the long run, according to David Glazier, an associate professor at Loyola Law School in Los Angeles. Another option would be to create a national security court that could apply military or federal standards but keep intelligence sources and methods secret.

Just before he was nominated attorney general last year, Michael Mukasey wrote an opinion column saying a national security court deserves “careful scrutiny by the public, and particularly by the U.S. Congress.” He also suggested looking at a proposal to lock up suspected terrorists using legal norms that allow the insane to be involuntarily committed.

The Supreme Court’s latest ruling gave all detainees the right to petition federal judges for immediate release. In a separate case for an individual detainee, a federal appeals court on Monday decided he was not an enemy combatant and ordered the military to release him, transfer him or hold a new proceeding promptly.

Commanders on this 45-square-mile base encompassing arid hills and a broad bay say they are ready to move the prisoners out if given the order.

Flexibility is literally built in. If Washington decides the war crimes trials should be moved to the U.S., a new high-tech courthouse and related facilities built on an abandoned airfield here can be dismantled and shipped over.