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A key document detailing TSA airport security screening procedures was accidentally posted online. Ms. Napolitano appeared to allude to this incident Sunday when she said screeners were rotating procedures at airports so as not to be predictable.

Republican leaders said the Northwest Airlines incident on Christmas seems to be the most egregious example of a security failure.

“There is much to investigate here. It’s amazing to me that an individual like this, who was sending out so many signals, could end up getting on a plane going to the U.S.,” Senate Minority Leader Mitch McConnell, Kentucky Republican, said on ABC’s “This Week.”

Other White House aides defended the response and broader efforts by the Obama administration to refocus the war on terror, including drawing down fighting forces in Iraq and refocusing efforts in Afghanistan and Pakistan.

“I think pretty quickly the White House determined, and we told many in the media and you all reported, that we believe this was a potential terrorist attack that, that could have occurred. The president certainly has taken steps in his time in office to reorient our priorities as it comes to fighting that war on terror,” Mr. Gibbs said.

Meanwhile Sunday, another incident involving a Nigerian occurred on a Northwest Amsterdam-Detroit flight. It was apparently just a coincidence, but the jitters from Friday’s incident prompted authorities to take the plane to a remote part of Detroit Metropolitan Airport to unload and screen the passengers.

A second Nigerian man, whom authorities did not immediately identify, was detained after locking himself in the plane’s bathroom and acting belligerently. Officials said that his claim of sickness turned out to be genuine.

The Associated Press reported, however, citing a government report it had obtained, that no air marshals were aboard Sunday’s Amsterdam-Detroit flight despite government promises of tighter security and mobilization of air marshals.

U.S. authorities said they think Mr. Abdulmutallab tried to ignite PETN, a nitroglycerin-related powder, in his underwear, and possibly a glycol-based liquid explosive in a syringe strapped to his leg.

The attempt set off popping, smoke and some fire but no major explosion. The crude detonator reportedly failed to get a proper blast out of the mixture.

Law enforcement officials told AP on the condition of anonymity that bomb-sniffing dogs, a human frisking or airport “puffer” machines, which blow air onto a passenger to collect and analyze residues, probably would have detected PETN powder. However, most airplane passengers go only through magnetometers, which detect metal rather than explosives.

Mr. Abdulmutallab was released to federal marshals Sunday and is now detained in Michigan on federal charges of attempting to destroy an aircraft and placing a destructive device in an airplane. He had been in a Michigan hospital receiving treatment for burns.