Despite the billions spent since 2001 on intelligence and counterterrorism programs, sophisticated airport scanners and elaborate watch lists, it was something simpler that averted disaster on a Christmas Day flight to Detroit: alert and courageous passengers and crew members.

During 19 hours of travel, aboard two flights across three continents, law enforcement officials said, Umar Farouk Abdulmutallab bided his time. Then, just as Northwest Flight 253 finally began its final approach to Detroit around noon on Friday, he tried to ignite the incendiary powder mixture he had taped to his leg, they said.

There were popping sounds, smoke and a commotion as passengers cried out in alarm and tried to see what was happening. One woman shouted, “What are you doing?” and another called out, “Fire!”

And then history repeated itself. Just as occurred before Christmas in 2001, when Richard C. Reid tried to ignite plastic explosives hidden in his shoe on a trans-Atlantic flight, fellow passengers jumped on Mr. Abdulmutallab, restraining the 23-year-old Nigerian.