In search for clues to the foiled Christmas Day bombing of a U.S.-bound plane, part of an explosive device was found sewn into the underwear of the Nigerian suspect, according to media reports quoting a U.S. law enforcement official on Monday.

A slightly charred and singed underpants with the explosive packet removed from the crotch is seen in government photos obtained exclusively by ABC News, released to Reuters, December 28, 2009.[Xinhua]

The U.S. security authorities have focused their investigation on how AbdulMutallab, 23, smuggled the explosives aboard the flight.

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.

According to the Netherlands' national coordinator for counterterrorism, AbdulMutallab had gone through "normal security procedures" before boarding the flight to Detroit.

AbdulMutallab, who had a multiple-entry visa to the United States, was in Yemen between August and December.

Relatives of the suspect said Monday that they told authorities weeks ago about AbdulMutallab's abnormal behavior. AbdulMutallab was studying abroad when he "disappeared" and stopped communicating with his relatives.

His father, Umaru AbdulMutallab, contacted Nigerian security agencies two months ago and foreign security agencies six weeks ago.

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

The package of the PETN explosive powder is seen in government photos obtained exclusively by ABC News, released to Reuters, December 28, 2009.[Xinhua]?

Al Qaeda on Monday claimed responsibility for the attack. In a message written in Arabic and published on radical Islamist Web sites, the group hailed the "brother" who carried out the "heroic attack." "There was a technical problem that resulted in a non-complete explosion," the message said.

U.S. investigators have not determined whether the al Qaeda claim of responsibility was true.

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

Meanwhile, the security is tighthened in the wake of the incident, triggering long lines at security checkpoints at airports in the United States and abroad. Airlines and their crews have been required to implement a "one-hour rule," which prohibits passengers from leaving their seats during the last hour of flight, sources said.

The syringe detonator is seen in government photos obtained exclusively by ABC News, released to Reuters, December 28, 2009.[Xinhua]

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 trying to blow up a Delta/Northwest flight flying from Amsterdam to Detroit with 289 people on board on Dec. 25.