President Barack Obama said Monday that the US would press ahead with its offensive against terror cells worldwide, just minutes after an al-Qaida-affiliated group in Yemen claimed responsibility for the airplane bombing attempt over Detroit on Christmas Day.

WASHINGTON — President Barack Obama said Monday that the U.S. would press ahead with its offensive against terror cells worldwide, just minutes after an al-Qaida-affiliated group in Yemen claimed responsibility for the airplane bombing attempt over Detroit on Christmas Day.

"This was a serious reminder of the dangers we face and of the nature of those who threaten our homeland," Obama said in his first comments about the incident aboard a Northwest Airlines flight from Amsterdam. "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.

"We will continue to use every element of our national power to disrupt, to dismantle and defeat the violent extremists who threaten us — whether they are from Afghanistan or Pakistan, Yemen or Somalia, or anywhere where they are plotting attacks."

Obama's remarks, delivered at a Marine base near his Hawaii vacation home, also marked the first time the administration has indicated it does not believe Umar Farouk Abdulmutallab acted alone.

The 23-year-old Nigerian managed to smuggle a packet of highly explosive PETN aboard the flight, along with some liquid in a syringe that he used as a detonator, authorities said. After the explosives caught fire, he was overpowered by passengers and crew.

"We will not rest," Obama said, "until we find all who were involved and hold them accountable."

Also on Monday, Homeland Security Secretary Janet Napolitano acknowledged that the suspect — who had a valid U.S. visa — had evaded measures meant to identify potentially dangerous travelers and detect explosives.

After discussions with the White House, Napolitano took to the airwaves to try to minimize criticism over remarks a day earlier in which she emphasized how the security system had worked after the incident — rather than dwelling on the failure to keep Abdulmutallab off the plane.

When asked on NBC's "Today" show Monday whether the system had "failed miserably," she answered: "It did."

"No one is happy.... An extensive review is under way," she said.

In his remarks, Obama outlined more aggressive security measures that were being taken, including enhanced screening and more federal air marshals on international flights. He said an investigation had been ordered to "determine just how the suspect was able to bring dangerous explosives aboard an aircraft and what additional steps we can take to thwart future attacks."

Even before Friday's attack, the United States — in conjunction with the Yemeni government — had stepped up its counter-terrorism operations in the country, seeking to combat a rapidly expanding al-Qaida network there.

Abdulmutallab has told authorities that the terrorist organization trained him and provided the explosives.

The Yemen foreign ministry confirmed Monday that Abdulmutallab visited several times — ostensibly to study Arabic at a school in San'a, the capital — including one trip from early August until early December of this year.

"Authorities are currently investigating who he was in contact with," the ministry said. But one Yemen official said it might be hard to trace Abdulmutallab's steps given how many students come from all over the world to the country to study Arabic and Islam.

Monday brought numerous developments, including a statement from the group al-Qaida in the Arabian Peninsula claiming the airline attack was retaliation for airstrikes Dec. 24 against suspected militants.

Anwar al-Awlaki, a radical U.S.-born cleric. may have been among those killed. Awlaki had communicated with the accused Fort Hood gunman before the attack in Texas last month that left 13 people dead.

In its communique, Qaida affiliate said Abdulmutallab had coordinated the plot with members of its group, using explosives they manufactured. The Web site posting was titled "The Brother Mujahid Omar Farooq al-Nigeri's Operation," and it included a photograph of a smiling Abdulmutallab in front of a Qaida banner.

The statement also boasted that Abdulmutallab had "infiltrated all the advanced, new machines and technologies and the security boundaries in the world's airports ... and he made all of what they spent on security development techniques a (new) heartbreak for them."

While acknowledging that the attack had not achieved its goal, the statement said, "We will continue on this path until we achieve success," according to a translation posted by the NEFA Foundation, an organization of counter-terrorism specialists.

A U.S. counterterrorism consultant said the communique appeared to be authentic.

In Amsterdam Monday, authorities said they were investigating whether an accomplice had helped Abdulmutallab board Flight 253 without a passport — possibly by claiming he was a Sudanese refugee.

In Nigeria, authorities continued to gather information after interviewing Abdulmutallab's family and friends and searching several locations in the expanding global investigation.

In Britain, Scotland Yard was looking into who might have helped radicalize Abdulmutallab during his years as an engineering student there ending in 2008.

In Detroit, a scheduled hearing in Abdulmutallab's case was canceled without explanation. But prosecutors continued their efforts to get a DNA sample from him to match against evidence recovered from the plane.

And in Washington, the Senate Homeland Security and Governmental Affairs Committee announced it would investigate lapses in airport security.

"What we know about the Abdulmutallab case raises two big, urgent questions.... Why aren't airline passengers flying into the U.S. checked against the broadest terrorist database, and why isn't whole body scanning technology that can detect explosives in wider use?" Sen. Joe Lieberman, I-Conn., said.

Congressional staffers also said the committee would investigate what steps U.S. officials took to investigate Abdulmutallab after his father — a respected Nigerian banker — shared concerns six weeks ago that his son's radicalized behavior and ties to extremists could pose a threat.

A State Department official said Monday that the father was "frantic" that Abdulmutallab had gone to Yemen, and had been hoping U.S. officials could help find his son.

The suspect's name was added to a broad informational database of 550,000 possible threats, but not to other warning lists that would have either barred him from traveling to the United States, subjected him to more intensive searches and questioning or even led to cancellation of his U.S. visa.

Republicans lashed out at the Obama administration Monday for what they called lapses in security and an inadequate response to the Christmas attack.

Rep. Peter T. King, R-N.Y., said Obama had waited too long to address the nation, missing a crucial opportunity to explain the threat and prepare air travelers for the inconvenience of intensified security checks.

He also blasted Napolitano for giving "contradictory messages" on the incident itself and the larger threat posed by al-Qaida and affiliated militants.

Administration officials conceded that Napolitano's Sunday comments had touched off a controversy, prompting a high-level discussion later in the day about how the secretary could "clarify" her remarks.

But with Congress preparing to hold hearings on the terrorism incident, Obama adviser David Axelrod said he hoped Republican lawmakers would not try to exploit it for partisan advantage. "Several of them were out there bashing before they even got briefed," Axelrod said. "There are people who want to turn every issue into a partisan issue. That's a shame."

Axelrod said the administration long had focused on Yemen as an emerging terrorist trouble spot. "Our people are well aware of the threat there," he said.

Paul Richter in Washington and Alana Semuels in Hawaii contributed to this report.