Once on the plane, Abdulmutallab walked to seat 19A, next to the wing, and above the fuel tank, apparently choosing the location to maximize the chances of bringing the plane down.
He wanted to blow up the plane over the airport so investigators would find evidence that it was terrorism, the sources said. He went into action just before landing, going to the bathroom and staying there for 20 minutes.
He came back to his seat complaining of a stomachache and covered himself with a blanket. Soon, passengers started noticing fire emitting from his pants.
"There was a bang," passenger Elias Fawaz recalled. "Sounded at first like a balloon went off."
As Abdulmutallab was trying to detonate the bomb in his lap, the wall of the plane caught fire, passengers said.
"I'm sitting here, the flames were leaping up at least this high," said Daniel Huisinga, putting his hands above his head.
The Nigerian national had packed 80 grams of the explosive pentaerythritol tetranitrate, or PETN, the same kind that shoe-bomber Richard Reid used, but even more powerful. But the explosives sewn into Abdulmutallab's underwear failed to ignite, and passengers subdued Abdulmutallab.
One passenger recalled the flight attendant's emotional announcement when the nightmare was over.
"He was just shaking and crying in the speaker, saying, 'The situation is taken care of, the fire is out,'" passenger Richelle Keepman said.
Napolitano said the administration is beginning to deploy new technology at airports.
"There will be lessons learned that we deal with and fix. And that process is ongoing, and as you might imagine, it's ongoing at lightning speed," she said on "GMA." "We are going to get to the bottom of this."
Some experts said while the United States already has new technology to combat this issue, it's not available nationwide, and it's very expensive and considered by some to be intrusive.
"Fewer than 5 percent of the screening posts at airports have the technology necessary to find this sort of thing. The technology exists, but it costs a lot of money, and it's very intrusive and some people think that it invades their privacy," ABC News consultant and former counterterrorism official Richard Clarke said on "GMA." "If we had this expensive, intrusive equipment at all screening posts, we might have been able to stop this."
Abdulmutallab's family had warned U.S. authorities of the increased radicalization of their son, a student at a London university until 2008. His father, a former Nigerian banker, had become concerned about his son's disappearance and lack of communication while studying abroad. He first contacted Nigerian security agencies about two months ago, and then some foreign security agencies about a month and a half ago, to seek assistance in finding and returning him home.
"It was while we were waiting for the outcome of their investigation that we arose to the shocking news of that day," the family said in a statement. "The disappearance and cessation of communication which got his mother and father concerned to report to the security agencies are completely out of character and a very recent development, as before then, from very early childhood, Farouk, to the best of parental monitoring, had never shown any attitude, conduct or association that would give concern. As soon as concern arose, very recently, his parents reported it and sought help."