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Even so, airport security and intelligence played no role in thwarting the plot. Mr. Abdulmutallab was carrying PETN, also known as pentaerythritol, the same material convicted shoe bomber Richard Reid used when he tried to destroy a trans-Atlantic flight in 2001 with explosives hidden in his shoes. Mr. Abdulmutallab is alleged to have carried the explosive in condomlike pouches attached to his body.

Mr. Abdulmutallab was on a “generic” terrorist watch list, which includes more than half a million names, but was not elevated to a no-fly list or even designated for additional security searches, Ms. Napolitano said. That would have required “specific, credible, derogatory information,” she said.

“We did not have the kind of information that under the current rules would elevate him,” she said.

Ms. Napolitano said the Obama administration is considering changing those rules.

Despite being on the broad terrorist watch list, Mr. Abdulmutallab, who comes from a prominent and wealthy Nigerian family, had a multiple-entry U.S. visa that was issued last year. U.S. officials say he came to the attention of American intelligence in November when his father expressed concerns to the U.S. Embassy in Nigeria about his son’s extremist views.

Ms. Napolitano said Mr. Abdulmutallab was screened properly before getting on the flight to Detroit in Amsterdam.

The administration is also investigating aviation detection systems to see how the alleged attacker managed to get on board the Northwest flight in Amsterdam with explosive materials, White House spokesman Robert Gibbs said.

No other flights were known to have been targeted. However, Mr. Gibbs said, federal authorities took precautionary steps “to assume and plan for the very worst.” Ms. Napolitano said there is no indication yet Mr. Abdulmutallab is part of a larger terrorist plot, although his possible ties to al Qaeda are still under investigation.

The United States is reviewing what security measures were used in Amsterdam where he boarded the flight.

“Now the forensics are being analyzed with what could have been done,” Ms. Napolitano said.

Additional security measures are in place at airports around the world that are likely to slow travelers. Ms. Napolitano advised getting to airports earlier.

Congress is preparing to hold hearings on what happened and whether rules need to be changed.

“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.,” said Senate Minority Leader Mitch McConnell, Kentucky Republican.

Mr. Gibbs appeared on ABC’s “This Week,” NBC’s “Meet the Press” and CBS’ “Face the Nation.” Ms. Napolitano spoke on CNN’s “State of the Union” as well as on NBC and ABC. Mr. McConnell appeared on ABC.

Pamela Hess reported from New York. Associated Press writers Ed White in Detroit and Devlin Barrett in Washington contributed to this report.