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“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,” said Mr. Obama, who is spending the holidays in Hawaii. “We will continue to use every element of our national power to disrupt, to dismantle and defeat the violent extremists who threaten us.”

On Capitol Hill, Sen. Joe Lieberman, Connecticut independent and chairman of the Homeland Security and Governmental Affairs Committee, said he will call a January hearing to investigate the handling of U.S. terrorist suspect lists and why body-imaging scanners are not used on more passengers.

Dutch airport authorities told Reuters news agency that the more-sensitive scanners likely will be made mandatory. Mr. Abdulmutallab purportedly smuggled powder explosives, which metal detectors cannot reveal, in his underwear through Schiphol Airport security in Amsterdam.

Also on Monday, Homeland Security Secretary Janet Napolitano said her statements a day earlier that “the system worked” were taken out of context.

“The whole process of making sure that we respond properly, correctly and effectively went very smoothly,” Ms. Napolitano also said Sunday on CNN’s “State of the Union.”

Ms. Napolitano said Monday that she had been referring to the government’s success in contacting and notifying other airports and aircraft immediately upon learning of the plot on Christmas Day.

She conceded during television interviews with several networks Monday that the system failed badly in preventing a potential devastating attack.

“Our system did not work in this instance,” she said on NBC’s “Today” show. “No one is happy or satisfied with that. An extensive review is under way.”

Domestic and international travelers described widely differing levels of security to reporters at U.S. airports Monday.

Some airline officials told the Associated Press and Reuters that the in-flight restrictions had been eased, giving captains discretion on matters such as whether passengers can have blankets or other items on their laps and can move about the aircraft in the last hour of the flight. But a Transportation Security Administration spokeswoman declined to confirm that.

Victor Morton contributed to this report.