HONOLULU — President Barack Obama said Monday he is pressing officials to determine how a man managed to board an airliner Christmas Day with an explosive device.

“We will not rest until we find all who were involved and hold them accountable,” Obama said. Calling Abdulmutallab’s action an “attempted act of terrorism,” Obama said he has asked his advisers to look into how 23-year-old Umar Farouk Abdulmutallab might have gotten an explosive aboard Northwest Airlines Flight 253 bound for Detroit from Amsterdam on Friday.

Obama said that upon being informed of the attempted terror attack he directed that immediate steps be taken to ensure the safety of the traveling public, including adding federal air marshals to flights leaving and arriving in the country.

Obama said he also has instructed the government to review current terrorism watch list policies system and of all technologies and procedures related to air travel. Abdulmutallab was on a general counterterrorism watch list that contains about 550,000 names, which is shared with airlines and foreign security agencies, but not on any sort of no-fly list.

“This was a serious reminder of the dangers we face and of the nature of those who threaten our homeland,” Obama said.

Homeland Security Secretary Janet Napolitano conceded Monday that the aviation security system failed, backtracking from a statement Sunday in which she said it worked.

The dangerous explosive allegedly concealed by Abdulmutallab in his underwear could have blown a hole in the side of the 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. 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 Reid.

Also Monday, 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.

Law enforcement officials in the United States, Yemen, Nigeria and the Britain spent a fourth day tracking the contacts and travel of Abdulmutallab, who is incarcerated on terrorism-related charges in Milan, Mich., until a bond hearing next week.

Members of Congress, meanwhile, questioned how a man flagged as a possible terrorist managed to board a commercial flight into the United States carrying powerful explosives and nearly bring down the jetliner. Sen. Joe Lieberman, I-Conn., said Monday that the Senate Homeland Security and Governmental Affairs Committee that he chairs would hold hearings in January.