Despite extensive spending 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, Abdul 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.

And then history repeated itself. Just as occurred before Christmas in 2001 when Richard C. Reid tried to ignite the plastic explosives hidden in his shoe on a trans-Atlantic flight, fellow passengers jumped on Mr. Abdulmutallab, restraining the 23-year-old Nigerian. Crew members poured bottled water on the flames, snuffing out the sparks of what could have been a planewide conflagration.

That close call was followed by several tense hours as counterterrorism officials checked on other United States-bound flights to determine whether more planes were targets, as in the thwarted 2006 plot to smuggle liquid explosives aboard multiple flights leaving from the United Kingdom.

They found no immediate signs that other flights were in danger, officials said. They tightened airport security but did not elevate the nation’s overall threat level, which has been at orange since 2006.

Although transportation officials had not announced new security measures yet, Air Canada said the Transportation Security Agency would make significant changes to the way passengers are able to move about on aircraft. During the final hour of flight, customers will have to remain seated, will not be allowed access to carry-on baggage and cannot have personal belongings or other items on their laps, according to a notice on Air Canada’s Web site.

In effect, that means passengers on flights of about 90 minutes or less will not be able to get out of their seats, since they are not allowed to move about while an airplane is climbing to its cruising altitude.

Air Canada also told its United States bound customers that they would be limited to a single carry-on item and that they would be subjected to personal and baggage searches at security check points and in the gate area. It said this would result in significant delays, canceled flights and missed connections. Air Canada said it would waive the baggage fee for the first checked bag as a result of the new policy.

Dozens of investigators led by the Federal Bureau of Investigation were working Saturday to understand exactly how a passenger managed to get flammable powder and a syringe of chemicals aboard the flight and what the implications might be for air safety at a time of heavy holiday travel. Intelligence agencies were studying intercepted communications to see whether clues were missed and assess whether the airliner fire could presage more attacks.

In London, Scotland Yard was conducting searches of apartments around University College, where a university spokesman said a man they identified at Umar Farouk Abdulmutallab had studied mechanical engineering from September 2005 to June 2008.

Mr. Abdulmutallab, who was sitting on the left side of the aircraft, may not have been trying to blow up the plane, but possibly intended to cause a fire to distract the cabin crew, cause passengers to panic and create a stampede for the exits as the plane was landing, a senior airline industry official who had been briefed on the situation said Saturday.

One passenger who was sitting in the same row as Mr. Abdulmutallab, but on the right side of the plane, apparently hurled himself across a middle row of four seats to tackle him. The passenger who hurtled across the aisle was Jasper Schuringa, a filmmaker from Amsterdam, a friend of Mr. Schuringa said Saturday. A senior airline executive said that flight attendants also jumped in.

Richard Griffith, 41, of Pontiac, Mich., said he had been sitting in the back of the plane when the commotion happened. Mr. Griffith praised the crew for its professionalism in averting the threat and a panic among passengers.

After landing, he said, he talked to the passenger who was sitting next to the suspect, in seat 19B. The passenger told him the suspect had done nothing during the flight other than get up once midflight to use the bathroom. Then, about 20 to 30 minutes before the incident, the suspect got up again, apparently to brush his teeth, he said.