SAN'A, Yemen - Former detainees of the U.S. prison in Guantanamo Bay, Cuba, have led and fueled the growing assertiveness of the al-Qaeda branch that claimed responsibility for the attempted Christmas Day bombing of a U.S. jet, potentially complicating the Obama administration's efforts to shut down the facility.

They include two Saudi nationals: al-Qaeda in the Arabian Peninsula's deputy leader, Said al-Shihri, and its chief theological adviser, Ibrahim Suleiman al Rubaish. Months after their release to Saudi Arabia, both crossed the porous border into Yemen and rejoined the terrorist network.

Both Shihri and Rubaish were released under the Bush administration, as was a Yemeni man killed in a government raid earlier this month while allegedly plotting an attack on the British Embassy. A Yemeni official said yesterday that the government thinks he was the first Yemeni to have joined al-Qaeda in the Arabian Peninsula after being released from Guantanamo.

That a group partly led by former Guantanamo detainees may have equipped and trained bombing suspect Umar Farouk Abdulmutallab is likely to raise more questions about plans to repatriate the Yemen prisoners.

Six were released last week, and 80 Yemenis are now at Guantanamo - nearly half the remaining detainee population. Many are heavily radicalized, with strong ties to extremist individuals or groups in Yemen, according to U.S. officials and terrorism analysts.

Republicans have in recent months urged the Obama administration to rethink sending detainees to Yemen. They have cited al-Qaeda's growing footprint in the country, its instability, and the case of Maj. Nidal Malik Hasan, the man charged with killing 13 people at Fort Hood, Texas, who had exchanged e-mail with a radical Yemeni American cleric.

"This is a very dangerous policy that threatens the safety and security of the U.S. people," said Rep. Frank Wolf (R., Va.).

A senior Obama administration official, speaking on condition of anonymity, said al-Qaeda had used Guantanamo "as a rallying cry and recruiting tool." Closing the prison, the official said, "is a national security imperative."

A second administration official said the government had little choice with the six detainees released last week. A federal judge had already ordered one released. The officials said the government concluded it lacked enough evidence to win against the remaining five in hearings in which the detainees had challenged their imprisonment under the doctrine of habeas corpus. The prospect of losing in federal court is likely to trigger other releases, the official said.

"We do not want a situation where the executive is defying the courts," the official said. "That's a recipe for a constitutional crisis."

Wolf, who did not object when the Bush administration repatriated 14 Yemeni detainees, said "conditions in Yemen have dramatically changed" with the emergence of al-Qaeda in the Arabian Peninsula.

Wolf added that he had access to classified biographies of the six Yemenis sent back last week. "Did they read the bios? They are dangerous people," Wolf said.

The Yemeni former Guantanamo detainee who joined al-Qaeda, Hani Abdo Shaalan, was among four suspects killed by Yemeni forces in a Dec. 17 raid, according to a Yemeni official and a rights activist. Shaalan, who was released from Guantanamo in June 2007, and three other suspected extremists were planning to bomb the British Embassy and other Western sites, said a Yemeni official who was briefed on the operation.

Shaalan, 30, had traveled to Afghanistan by way of Pakistan in July 2001, seeking work. He eventually found work as a chef's assistant in a Taliban camp and was at Tora Bora during the U.S. air campaign there. Pakistani forces captured him in their country, near the Afghan border.

Shaalan's family reported his disappearance last year, said Ahmed Amran, a human-rights lawyer who assists the repatriated detainees.

After their release from Guantanamo, Shihri and Rubaish, who both trained and fought with al-Qaeda in Afghanistan, were sent to a Saudi rehabilitation program that uses dialogue and art therapy to reform extremists. In February, the Saudi government released a list of 85 most-wanted Saudi extremists. At least 11 were graduates of the program, including Shihri and Rubaish.