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VIA: CNN.com

President Obama warned Monday that the United States would respond aggressively to terrorism such as last week’s botched attempt to blow up a U.S. airliner.

“Those who would slaughter innocent men, women and children must know the United States will do more than simply strengthen our defenses,” Obama said.

Obama said the government was doing “everything in our power to keep you and your families safe and secure during this busy holiday season.”

Al Qaeda in the Arabian Peninsula claimed responsibility for the attempted Christmas Day terrorist attack on a plane about to land in the U.S., saying it was in retaliation for alleged U.S. strikes on Yemeni soil.

In the statement, published on radical Islamist Web sites, the group hailed the “brother” who carried out the “heroic attack.” The group said it tested a “new kind of explosives” in the attack, and hailed the fact that the explosives “passed through security.”

The group threatened further attacks, saying, “since Americans support their leaders they should expect more from us.”

“We have prepared men who love to die,” the statement dated Saturday said.

A suspect, Umar Farouk AbdulMutallab, is being held for allegedly trying to blow up the flight carrying 300 passengers.

Part of the explosive device was sewn into AbdulMutallab’s underwear, a law enforcement official told CNN Monday.

A preliminary FBI analysis found that the device AbdulMutallab allegedly carried aboard the flight from Amsterdam, Netherlands, to Detroit, Michigan, contained the explosive pentaerythritol tetranitrate, known as PETN. The source could provide no details on the device.

The amount of explosive involved was sufficient to blow a hole in the aircraft, a source with knowledge of the investigation told CNN Sunday.

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.

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.

“Now, we are going to be looking at that process and how those lists are created, maintained, updated, exchanged and the like, because clearly this individual should not have been able to board this plane carrying that material,” Napolitano said.

Napolitano told CNN on Sunday there was no indication that the failed attack was part of any larger international terrorist plot.

Tighter security measures in the wake of the incident triggered long lines at security checkpoints at airports in the United States and abroad. President Obama has ordered a review of security procedures. Both the House and Senate plan to hold hearings on the incident.

AbdulMutallab’s family said Monday it had told authorities about his “out of character” behavior and hoped that authorities would intervene.

The 23-year-old suspect was studying abroad when he “disappeared” and stopped communicating with his family members, they said Monday in a statement. His father, Umaru AbdulMutallab, contacted Nigerian security agencies two months ago and foreign security agencies six weeks ago, the statement 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.”

The suspect’s family said his behavior prompted it to seek help.

“The disappearance and cessation of communication which got his mother and father concerned to report to the security agencies are completely out of character and a very recent development, as before then, from very early childhood, Farouk, to the best of parental monitoring, had never shown any attitude, conduct or association that would give concern,” his family said.

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.

The embassy — which has law enforcement, security and intelligence representatives on staff — reported the father’s concern to other agencies, the 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.

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.

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.

Over the weekend in Britain, where the suspect studied engineering at a London university, police searched AbdulMutallab’s last known address.

Scotland Yard detectives on Sunday interviewed Michael Rimmer, a former high-school teacher who described AbdulMutallab as a “very devout” Muslim who had once expressed sympathy for Afghanistan’s Taliban insurgency during a classroom discussion.

But Rimmer, who taught AbdulMutallab at a school in the west African nation of Togo, said it was not clear whether the then-teenager was simply playing devil’s advocate during the class.

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.”

Yemeni authorities said they will take immediate action once the attempted bombing suspect’s alleged link to the country is officially identified.