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Umar Farouk Abdulmutallab

WASHINGTON- A dangerous explosive allegedly concealed by Nigerian student Umar Farouk Abdulmutallab in his underwear could have blown a hole in the side of his Detroit-bound aircraft if it had been detonated, according to two federal sources briefed on the investigation.

Authorities said they are still analyzing a badly damaged syringe that Abdulmutallab allegedly employed as a detonating device on Christmas Day. But preliminary conclusions indicate that he used 80 grams of PETN - almost twice as much of the highly explosive material used by convicted shoe bomber Richard C. Reid.

A day after Homeland Security Secretary Janet Napolitano said there was "no indication" the incident was connected to a larger plot, there were increasing signs that the failed bombing may have represented one of the most serious terrorist threats in the United States since the attacks of Sept. 11, 2001.

President Obama broke from his vacation in Hawaii to declare that authorities "will not rest until we find all who were involved and hold them accountable." He also said he had ordered a review of the nation's terror watch-list system, saying the failed plot was "a serious reminder" of the need to continually adapt security measures against changing terrorist threats.

In a statement, al-Qaida in the Arabian Peninsula, a group based in Saudi Arabia and Yemen, claimed responsibility for the attempt to destroy the Northwest Airlines jet, saying it was a response to U.S.-backed airstrikes against the group in Yemen. Meanwhile, Yemen's government confirmed that Abdulmutallab was in the country from early August to early December 2009 after obtaining a visa to study Arabic at a language institute, and said that he had previously studied at the school.

The suspect, 23, has told federal investigators that he had ties to al-Qaida, and that he had traveled to Yemen to collect the incendiary device he tried to use on the plane.

The Obama administration suffered blistering criticism for another day from national security experts and Republican lawmakers who demanded changes to the airline screening system and the use of more intrusive technology to detect explosives. Napolitano acknowledged on NBC's "Today" show Monday that "our system did not work in this instance. No one is happy or satisfied with that."

Law enforcement officials in the United States, Yemen, Nigeria and Britain spent a fourth day tracking the contacts and travel of Abdulmutallab, who is incarcerated on terrorism-related charges in Milan, Mich., 45 miles south of Detroit, until a bond hearing next week. His public defender, Miriam Siefer, did not return calls for comment.

The statement of responsibility by al-Qaida in the Arabian Peninsula, if deemed credible by U.S. intelligence agencies, may mark the first time al-Qaida affiliates outside of Afghanistan or Pakistan have targeted American soil.

One of the top leaders of al-Qaida in the Arabian Peninsula is Said al-Shihri, 36, a Saudi national. He was captured in Pakistan in December 2001 and spent six years at the Guantanamo Bay, Cuba, facility before being transferred to Saudi Arabia in November 2007.

In Saudi Arabia, he entered a highly praised rehabilitation program that uses dialogue and art therapy to convince former militants to renounce extremism. But after graduating, Shihri crossed the border into Yemen and rejoined al Qaida.