The alleged Christmas day terrorism plot has complicated plans by Barack Obama, US president, to shut the Guantánamo Bay prison camp.

Following reports that two former inmates who took refuge in Yemen were behind the botched plot, senators from his own party were among those calling for detainee transfers to be halted pending an investigation into the links between Guantánamo and the Arab nation.

“Guantánamo detainees should not be released to Yemen at this time,” said Dianne Feinstein, the senior Democrat on the Senate intelligence committee. “It is too unstable.”

Kit Bond, the top Republican on the committee, went further, calling for a rethink of the entire policy. “The question is: are we going to take every step possible to keep our country safe? That means stop releasing Gitmo detainees now,” Mr Bond told The Hill newspaper.

As it seeks to fulfil his pledge to close down the prison camp, Mr Obama’s administration has been sending detainees back to their home countries, including six Yemenis who were repatriated the week before Christmas.

Even before the Christmas day incident, the administration was wary about transferring Yemenis because of concerns about the country’s instability. About 90 of the 198 inmates still in Guantánamo are from Yemen.

Fears increased after a Yemen-based group calling itself al-Qaeda in the Arabian Peninsula (AQAP) said it planned the attack in retaliation for US support of a Yemeni military strike on one of its bases, which killed up to 60 of its members. Umar Farouk Abdulmutallab, the 23-year-old Nigerian charged over the alleged plot, told investigators he was given explosives by al-Qaeda operatives in Yemen, where he had been studying Arabic.

The suspected masterminds of the alleged plot were Muhamad al-Awfi and Said Ali al-Shihri, Guan-tánamo inmates who were sent to Saudi Arabia in November 2007 but fled to Yemen in 2008, ABC News reported, quoting US officials.

Three senators – Republicans John McCain and Lindsey Graham, and independent Joseph Lieberman – who this week wrote to the White House saying that the transfers were “highly unwise and ill-considered”, described Mr Shihri as “AQAP’s longstanding deputy”. He is thought to have orchestrated a 2008 car bombing of the US embassy in Yemen but could have been killed in a December 24 air strike.

The senators said: “Many of the other leaders of AQAP were previously held in Yemeni government custody. However, they escaped in February 2006 from a maximum-security prison in Sana’a.”

The White House said Guantánamo had been used by al-Qaeda as a recruiting tool and must be closed.

The treatment of prisoners at Guantánamo Bay, particularly allegations of torture and misuse of the Koran, has caused outrage in the Muslim world and been used as a call to jihad, or holy war.

The White House insisted that it was taking seriously the signs that some detainees were returning to al-Qaeda groups. “We’ve been working this through very aggressively,” a senior official said, reiterating the “bottom line” that keeping Guantánamo open was bad for national security.

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