Napolitano said on NBC's Today Show that the 23-year-old Nigerian suspect, Umar Farouk Abdulmutallab, should not have been allowed on the airplane.

"What I would say is our system did not work in this instance — no one is happy or satisfied with that. An extensive review is under way," Napolitano said of the foiled attempt to blow up Northwest Flight 253 over Detroit.

On Sunday, she had declared that the security screening system had worked.

"This was one individual, literally, of thousands that fly and thousands of flights every year," Napolitano said on CNN. "And he was stopped before any damage could be done."

An apparent malfunction in a device designed to detonate the explosive material may have been all that saved the 278 passengers and the crew. No undercover air marshal was on board the flight, and passengers and crew subdued the suspect and tamped out a fire after he allegedly tried to set off the bomb.

The White House has ordered an investigation into the incident, Napolitano said.

Abdulmutallab had been put on a watch list in May after his father, a prominent Nigerian banker, reported concerns about his son's extreme religious views to the U.S. Embassy in Nigeria. The Terrorist Identities Datamart Environment database list contains approximately 550,000 names. Officials say they lacked enough evidence to place Abdulmutallab on a list of about 14,000 people who need additional screening or an even shorter no-fly list containing fewer than 4,000 names.

Airport detectors in Amsterdam, where the flight originated, also reportedly failed to pick up any trace of the packet of powder and a liquid-filled syringe that was reportedly sewn into Abdulmutallab's underwear. In Sunday's CNN interview, Napolitano would not confirm the nature of the explosives or how they were brought onto the plane.

If the explosive was a substance known as PETN, as has been reported, a metal detector would have missed it, but a dog might have been able to find it, NPR's Dina Temple-Raston said Monday.

"A pat-down wouldn't necessarily have detected it," she said. Security experts also said airport "puffer" machines that blow air on a passenger to collect and analyze residues would probably have detected the powder.

Although officials have said they do not think Abdulmutallab was part of a larger plot to blow up more airplanes, they were looking carefully at the explosive, which would have been difficult for an individual to obtain, Temple-Raston said.

"Having a chemical detonator is also a new wrinkle," she said. "That isn't something that a lone wolf is likely to come up with."

Abdulmutallab told investigators that he got the explosive and training to use the device in Yemen, from someone linked to al-Qaida. Officials haven't been able to confirm that.

Abdulmutallab had been refused entry to Britain and placed on a security watch list after he applied to study at a bogus college, Britain's Home Secretary Alan Johnson said Monday. But earlier, from 2005 until June 2008, Abdulmutallab had studied mechanical engineering at the high-ranking University College London.

Johnson said authorities are trying to establish what communication if any occurred between Britain and the U.S. He said security agencies are trying to determine what sort of activities Abdulmutallab engaged in while in the U.K., and when he became radicalized.

One of his former high school teachers at the British International School in Togo said Abdulmutallab expressed radical views as far back as 10th grade. Michael Rimmer recalled that after the Taliban destroyed ancient statues in Afghanistan, other Muslims in the class distanced themselves, but Abdulmutallab said he supported the Taliban's actions.

**From NPR's Scott Neuman, Barbara Bradley Hagerty, Ofeibea Quist-Arcton and Dina Temple-Raston