A U.S. soldier keeps watch from a guard tower overlooking Camp Delta detention centre at Guantanamo Bay, Cuba, in this photo taken in December 2006. (Brennan Linsley/Associated Press)

The alleged Yemeni roots of the bomb attack on Northwest Airlines Flight 253 are complicating U.S. efforts to close the Guantanamo Bay prison, where nearly half the remaining detainees are from Yemen.

Umar Farouk Abdulmutallab, a Nigerian accused of trying to blow up the plane, told U.S. authorities he was carrying out the attack on orders from al-Qaeda.

Al-Qaeda in Arabian Peninsula, which is based in Saudi Arabia and Yemen, has claimed Abdulmutallab co-ordinated with members of the group and that the explosives he allegedly carried on the flight were made by al-Qaeda members.

The organization is led by a Yemeni who escaped from a Yemeni prison in 2006 and two of the group's leaders in Yemen are Saudis who were released from Guantanamo in November 2007.

Officials in Yemen were investigating Tuesday whether Abdulmutallab spent time with al-Qaeda militants in the country, where he briefly attended a school to study Arabic.

While inmates of other nationalities have left Guantanamo in droves, roughly 90 Yemenis have been held there for as long as seven years.

Six detainees were sent home to Yemen from Guantanamo earlier this month in a rare transfer that was viewed as a trial run for others to come.

David Remes, an attorney who represents Guantanamo detainees, said he fears concerns about the terrorism threat will block the repatriation of any more 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."

Steven Emerson, executive director of the Investigative Project on Terrorism research group, said he wouldn't be surprised if 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."

The U.S. has expressed concern about the handling of militants in Yemen, a mountainous, impoverished country that has been an al-Qaeda haven partly because of a weak central government.

With files from The Associated Press