At least one leader of al-Qaeda's branch in Yemen, where the failed bomber of a U.S.-bound Christmas flight was allegedly trained, was freed from the U.S. prison in Guantanamo, Cuba, according to a Pentagon list released in May.

The list names 27 former prisoners who resumed terrorist activities after being released from Guantanamo, including Said Ali al-Shihri, who was transferred to Saudi Arabia in 2007 and later implicated in the 2008 bombing of the U.S. embassy in Yemen's capital Sana'a.

ABC Television named former al-Qaeda leader in Yemen Muhammad Attik al-Harbi as another unrepentant former Guantanamo prisoner.

Yemen's role as an al-Qaeda haven has come under renewed public scrutiny in the wake of a 23-year-old Nigerian's failed attempt to detonate a bomb on Dec. 25 on a Northwest Airline plane as it approached to land in Detroit.

Umar Farouk Abdulmutallab, who boarded the plane in Amsterdam, was overpowered by passengers and crew and arrested on landing in Detroit.

American law enforcement officials, quoted anonymously by U.S. media, have said Mr. Abdulmutallab confessed to receiving specific training for the attack from an al-Qaeda bombmaker in Yemen.

An al-Qaeda affiliate in the Arabian peninsula claimed yesterday it was behind the failed bombing and threatened new attacks on the West, U.S. monitoring groups said.

The two-page statement, which was accompanied by a picture of Mr. Abdulmutallab, boasted that the "Nigerian brother ... was able to breach all the modern and sophisticated technologies and checkpoints at the airports around the world."

The U.S. government, however, remains cautious in linking the suspect to al-Qaeda.

Al-Qaeda has claimed responsibility for the Sept. 11, 2001 attacks on the United States. It has strengthened its presence in Yemen by exploiting the loose control of the central government over the heavily tribalized provinces.

Washington identified Yemen as a potential al-Qaeda stronghold soon after the Sept. 11 attacks and established a counterterrorism base just across the Bab al-Mandab strait in Djibouti to run operations in the region.