Finding a home for them is key to President Obama's pledge to close the prison, but emerging details of the plot are renewing concerns about Yemen's capacity to contain militants and growing al Qaeda safe havens.
While inmates of other nationalities have left Guantanamo in droves, roughly 90 Yemenis have been held at the U.S. military prison in Cuba for as long as seven years.
A breakthrough seems less likely since al Qaeda in the Arabian Peninsula claimed responsibility for the plot to bomb a U.S.-bound airliner on Christmas Day. The group counts two former Guantanamo detainees among its leaders, and some in the U.S. Congress are warning against sending any more detainees to Yemen.
David Remes, an attorney who represents Guantanamo detainees, said he fears concerns about the terror threat will block the repatriation of any inmates to Yemen, including those already cleared for release.
"In theory, what's going on in Yemen should have nothing to do with whether these men are transferred," he said. "The politics of the situation may turn out to be prohibitive, at least in the short run, and that would be a tragedy."
The U.S. has expressed concern about the handling of militants in Yemen, a mountainous, impoverished country on the southern tip of the Arabian peninsula that has been an al Qaeda haven partly because of a weak central government.
On Tuesday, officials in Yemen were investigating whether the Nigerian suspected in the attempted attack on a U.S. airliner spent time with al Qaeda militants in the country, where he briefly attended a school to study Arabic.
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Al Qaeda in the Arabian Peninsula, which was created in a merger between operatives from Saudi Arabia and Yemen, is led by a Yemeni who escaped from a Yemeni prison in 2006 with 22 other al Qaeda figures. And two of the organization's leaders in Yemen are Saudis who were released from Guantanamo in November 2007.
Steven Emerson, the executive director of the Investigative Project on Terrorism research group, said he would not be surprised if those former Guantanamo detainees were behind the airliner attack.
"Serving time in Gitmo has become a status thing for al Qaeda terrorists," Emerson said. "Those that have served time have become appointed to top positions within the terrorist group once they make their way back to Yemen."
from Guantanamo earlier this month in a rare transfer that was viewed as a trial run for others to come. But Remes said he expects optimism to fade among Yemeni detainees, men he describes as effectively "stateless."
A task force created by Obama has been reviewing each Guantanamo detainee's file to determine whether they should be prosecuted, detained or transferred. U.S. officials have declined to reveal details of any discussions with Yemen.
A senior administration official said authorities still see closing the facility as a national security priority. While Obama has directed the U.S. to acquire ato hold as many as 100 Guantanamo detainees, he is counting on sending others back to their homelands or, in cases where that is impossible, to willing third-party countries.
While detainee transfers to Yemen are likely to face closer scrutiny, the U.S. has also begun working closely with Yemen to fight terrorism, providing $70 million in military aid this year.
Sheila Carapico, a Yemen expert at the University of Richmond, said joint military operations against al Qaeda sites suggest cooperation at high levels that could facilitate an agreement to transfer and monitor Guantanamo detainees.
"This suggests to me a whole new era of cooperation, which will probably include discussions of what to do with these Gitmo guys," she said.
Yemen has said publicly that it wants all its nationals sent home from Guantanamo.