MIAMI — The foiled Christmas Day plot to blow up a jetliner over Detroit has thrown up a major roadblock to President Barack Obama's pledge to close the prison camps at Guantanamo Bay.

Cascading reports that the alleged would-be bomber, Umar Farouk Abdulmutallab, had trained in Yemen, and that the plot was hatched by two former Guantanamo detainees, have even supporters of emptying the prison predicting a new impediment to their effort.

Nearly half of the 198 captives at Guantanamo are citizens of Yemen — also the ancestral home of Osama bin Laden — just south of Saudi Arabia.

"This is just disastrous for the Yemenis at Guantanamo," said Washington, D.C., attorney David Remes, who over the years has defended 17 Yemeni detainees, some now slated for release.

Remes said an Obama administration task force has cleared for release "as many as 40 Yemenis" declared unfairly held or no longer threats, but that "the politics of the situation may make it impossible for the administration to send any Yemenis back to Yemen in the foreseeable future."

Yemen also was a thorny issue for the Bush administration, which tried but failed to establish a safe tracking and rehabilitation system for Guantanamo detainees returned there.

The White House tested the water earlier this month by sending six Yemenis home, the largest transfer to the nation since Guantanamo opened eight years ago.

Now, the Christmas Day bomb attempt has renewed calls to maintain Guantanamo as a detention center, even as the White House is trying to win congressional approval to purchase an unused state prison in northern Illinois to hold detainees facing trial by military commission and some whom cannot be released.

"I know the president made a promise that he'd close Guantanamo because of what it represented in world opinion," U.S. Sen. Joe Lieberman, a Connecticut independent, told "Fox News Sunday."

"But today it's a first-class facility," he said of the remote prison that holds captives from 28 nations in seven different camps. "It would be a mistake to send these 90 people back to Yemen, because based on the past of what's happened when we've released people from Guantanamo, a certain number of them have gone back into the fight against us."

Indeed, the Bush administration freed or sent home nearly 500 detainees, among them two Saudi Arabian men that ABC News said were behind Abdulmutallab's ill-fated effort to blow up the airliner with explosives hidden in his underpants.

In contrast, the Obama administration to date has transferred only 44 detainees from Guantanamo.

Of those, three face criminal trials in Italy and New York. A fourth was taken for burial in Yemen, a suspected suicide after he was discovered dead of undisclosed causes in the prison psychiatric ward.

One of Obama's first acts as president was to order the detention center closed, saying it had served as a recruiting tool for al-Qaida around the world.

The White House has acknowledged in recent months that it won't meet the president's Jan. 22 deadline for closure. But on Tuesday, it was sticking to the blueprint it developed in the intervening year of releasing some captives, moving others to trial and holding yet others in a state of indefinite detention approved by the courts through habeas corpus challenges.

"The president will not release any detainee who would endanger the American people," a senior administration official told The Miami Herald Tuesday, on condition he not be named because he was not authorized to speak about the topic. "We have worked cooperatively with the government of Yemen to ensure that all appropriate security measures are taken when detainees are transferred." Officials say the White House is counting on congressional approval for its Illinois prison plan and that it has full confidence in Obama's Guantanamo Review Task Force, saying that the process is more thorough than the Bush administration's, which resulted in the release of two detainees now linked to the Detroit airliner plot.

ABC News identified the men as Mohamed al-Harbi and Said al-Shihri, both Saudi nationals who were repatriated to a rehabilitation program in the oil-rich kingdom in December 2007. They have re-emerged as leaders of an al-Qaida offshoot in Yemen.

What will become of the Yemenis now held at Guantanamo is not clear.

Remes suggested that the State Department, which has won resettlement of Palestinians, Algerians and Uighurs to Europe and elsewhere, may add cleared Yemenis to those for whom it is seeking asylum.

An architect of Bush-era detention policy, retired Navy Cmdr. Kirk Lippold — who saw 17 of his sailors killed in the October 2000 al-Qaida attack on the USS Cole off Yemen — seized on the latest developments to defend continued detention.

A policy of more releases from Guantanamo "presents an unacceptable risk to American lives," Lippold said in a statement issued by the Washington, D.C., lobby Military Families United. "As a nation, we cannot rely on so-called 'reform camps' in places like Saudi Arabia to prevent terrorists from striking again."

Middle East expert Christopher Boucek of the Carnegie Middle East Endowment for International Peace said Yemen in particular presents the Obama administration with the largest obstacle in its bid to close the camps.

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"If you want to solve Guantanamo, you have to solve the Yemen issue. To the best of my knowledge there has been no coherent plan for that," he said, starting with the fact that the government in Sana'a has yet to create a convincing rehabilitation program for former prisoners to re-enter society at a time when civil war is imploding the nation's already poor economy.

Recent Yemeni Air Force strikes on a suspected al-Qaida training camp outside the capital as well as security sweeps in Sana'a' show some resolve to work with the United States on subduing the threat from within, he said.

But in the end, there is no way to be certain that a former Guantanamo detainee won't end up engaging in terrorism.

"Something that has been missing about all the discussion of Guantanamo is that nobody has come out and said there will be people who reoffend, there will be people who are released who will do bad stuff. And the public is not being prepared for this," Boucek said