Romulus, Michigan (CNN) -- Part of an explosive device that failed to take down a plane last week was sewn into the underwear of the Nigerian man accused of igniting it, a law enforcement official told CNN Monday.

Umar Farouk AbdulMutallab is being held for allegedly trying to blow up a flight carrying 300 passengers on Christmas Day.

Al Qaeda in the Arabian Peninsula claimed responsibility Monday for the attack, saying it was in retaliation for alleged U.S. strikes on Yemeni soil.

In a message written in Arabic, dated Saturday and published Monday on radical Islamist Web sites, the group hailed the "brother" who carried out the "heroic attack."

The group said it tested "new kind of explosives" in the attack and hailed the fact that the explosives "passed through security."

"There was a technical problem that resulted in a non-complete explosion," the message said.

A preliminary FBI analysis found that the device AbdulMutallab is said to have carried aboard the flight from Amsterdam, Netherlands, to Detroit, Michigan, contained pentaerythritol tetranitrate, an explosive also known as PETN. The amount of explosive was sufficient to blow a hole in the aircraft, a source with knowledge of the investigation told CNN Sunday.

In his first public comment since the Christmas Day incident, President Obama said he directed his national security team to "keep up the pressure on those who would attack our country."

"We do not yet have all the answers about this latest attempt, but those who would slaughter innocent men, women and children must know that the United States will do more than simply strengthen our defenses," Obama told reporters in a break from his Christmas holiday in Hawaii.

U.S. investigators have not determined whether the al Qaeda claim of responsibility was true, but one U.S. counterterrorism official told CNN on Monday that the group might have some involvement.

CNN National Security Analyst Peter Bergen said that if al Qaeda operatives in Yemen were behind the Christmas plot, that would represent a significant advance for the group.

"Most of the attacks we have seen in the past have been in Yemen or Saudi Arabia, and the [al Qaeda] affiliate there has not been able to do out-of-the-area operation," Bergen said.

A federal security bulletin obtained by CNN said AbdulMutallab claimed the explosive device used Friday "was acquired in Yemen along with instructions as to when it should be used."

Yemen's government launched airstrikes and other raids against al Qaeda operatives on its territory over the past two weeks, and Monday's statement by al Qaeda accused the United States of assisting with cruise missile attacks launched from offshore.

U.S. officials privately acknowledge they have provided secret intelligence on several al Qaeda targets to Yemen's government but won't say whether U.S. aircraft or drones took part in the strikes.

Al Qaeda in the Arabian Peninsula threatened further attacks in its statement, saying, "We have prepared men who love to die."

"By God's permission, we will come to you with more things that you have never seen before," it said.

Yemen's Ministry of Foreign Affairs condemned Friday's attempted attack in a statement released Monday. "Yemen has long suffered from terrorism and condemns such criminal acts that kill innocent civilians. Yemen is and remains an active partner of the international community in the war against terrorism. Efforts of Yemeni security agencies to continue ongoing operations and prosecutions against terrorist operatives from al Qaeda will not falter," it said.

Mohammed Albasha, spokesman for the Yemen Embassy in Washington, confirmed to CNN Monday that AbdulMutallab was in Yemen between August and December. According to the ministry's statement, AbdulMutallab had obtained a visa to study Arabic at a language institute in Yemen where he had previously studied.

Relatives of the suspect said Monday that they told authorities weeks ago about AbdulMutallab's "out of character" behavior and hoped authorities would intervene.

AbdulMutallab, 23, was studying abroad when he "disappeared" and stopped communicating with his relatives, they said in a statement. His father, Umaru AbdulMutallab, contacted Nigerian security agencies two months ago and foreign security agencies six weeks ago, the statement said.

A senior U.S. administration official said one of those agencies contacted was the U.S. Embassy in Nigeria. The embassy, which has law enforcement, security and intelligence representatives on staff, reported the father's concern to other agencies, the official said.

"We were hopeful that they would find and return him home," the family said. "It was while we were waiting for the outcome of their investigation that we arose to the shocking news of that day."

AbdulMutallab, a Nigerian who had a multiple-entry visa to the United States, had been added to a watch list of 550,000 potential terrorist threats after the information provided by his father was forwarded to the National Counter-Terrorism Center, a senior administration official said. But "the info on him was not deemed specific enough to pull his visa or put him on a no-fly list," the official said.

Obama said Monday he has "ordered a thorough review, not only of how information related to the subject was handled, but of the overall watch-list system and how it can be strengthened."

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The father of the suspect contacted the U.S. Embassy in Nigeria with concerns his son had "become radicalized" and was planning something, a senior U.S. administration official said.

"After his father contacted the embassy recently, we coded his visa file so that, had he attempted to renew his visa months from now, it would have triggered an in-depth review of his application," a U.S. official said.

Passengers on the Christmas Day flight described a chaotic scene that began with a popping sound as the plane was making its final approach, followed by flames erupting at AbdulMutallab's seat.

The suspect was moved Sunday from a hospital, where he was treated for his burns, to an undisclosed location in the custody of the U.S. Marshals Service. He is charged with attempting to destroy the plane and placing a destructive device on the aircraft.

Authorities have focused their investigation on how AbdulMutallab, 23, allegedly smuggled the explosives aboard the flight and who might have helped him.

"We're ascertaining why it was that he was not flagged in a more specific way when he purchased his ticket, given the information that we think was available, allegedly was available," Homeland Security Secretary Janet Napolitano told CNN's "American Morning" Monday.

Meanwhile, tighter security measures in the wake of the incident triggered long lines at security checkpoints at airports in the United States and abroad. Airlines and their crews have been given discretion over implementation of a "one-hour rule," which prohibits passengers from leaving their seats during the last hour of flight, sources said. The Transportation Security Administration invoked the rule for international U.S.-bound flights after the botched attack.

Airline security will be the focus of hearings by the Senate committees on Homeland Security and Governmental Affairs and Commerce, Science and Transportation; the House Committee on Homeland Security is also slated to hold a hearing on the incident.

AbdulMutallab's trip originated in Lagos, Nigeria. There, he did not check in a bag as he flew on a KLM flight to Amsterdam, said Harold Demuren, director-general of Nigeria's Civil Aviation Authority.

Demuren said the suspect underwent regular screening -- walking through a metal detector and having his shoulder bag scanned through an X-ray machine.

He then underwent secondary screening at the boarding gate for the KLM flight, according to officials of the Dutch airline.

After arriving in Amsterdam, AbdulMutallab boarded the Northwest Airlines flight to the United States.

The Netherlands' national coordinator for counterterrorism told CNN that AbdulMutallab had gone through "normal security procedures" in Amsterdam before boarding the flight to Detroit.



CNN's Elise Labott, Jeanne Meserve, Carol Cratty, Richard Quest, Nic Robertson, Christian Purefoy, Tom Cohen, Mike Ahlers, Alona Rivord, Mohammed Jamjoom, Miguel Susana and Barbara Starr contributed to this report.