The chemical -- PETN -- is small, powerful and appealing to terrorists. The Saudi government said it was used in an assassination attempt on the country's counterterrorism operations chief in August.

It was also a component of the explosive that Richard Reid, the convicted "shoe bomber," used in his 2001 attempt to down an airliner.

PETN was used widely in the plastic explosives terrorists used to blow up airplanes in the 1970s and '80s.

Investigators say Umar Farouk Abdulmutallab hid an explosive device on his body when he traveled from Amsterdam to Detroit. They say PETN was hidden in a condom or condomlike bag just below his torso.

Mr. Abdulmutallab also had a syringe filled with liquid. One law enforcement official said the second part of the explosive concoction used in the Christmas Day incident is still being tested but appears to be a glycol-based liquid explosive. The officials spoke on condition of anonymity because they were not authorized to discuss the investigation.

PETN is the primary ingredient in detonating cords used for industrial explosions and can be collected by scraping the insides of the wire, said James Crippin, a Colorado explosives expert. It's also used in military devices and found in blasting caps. It's the high explosive of choice because it is stable and safe to handle, but it requires a primary explosive to detonate it, he said.

Mr. Crippin and law enforcement officials said modern airport screening machines could have detected the chemical. Airport "puffer" machines -- the devices that blow air onto a passenger to collect and analyze residues -- would probably have detected the powder, as would have bomb-sniffing dogs or a hands-on search using a swab.

However, most passengers in airports only go through magnetometers, which detect metal rather than explosives.

Hidden in Mr. Abdulmutallab's clothing, the explosive might have also been detected by the full-body imaging scanners now making their way into airports.

But Mr. Abdulmutallab did not go through full-body imaging machines in Nigeria or Amsterdam, said U.S. Rep. Peter T. King, the top Republican on the Homeland Security Committee. Mr. King has been briefed on the investigation.

Both airports have body scanners. The Amsterdam airport long has had a reputation for good security, Mr. King said, while Nigeria's airports have been more of a concern.

The United States provided full-body scanners to all four international airports in Nigeria, according to the State Department. The scanners were installed in March, May and June of 2008.

Mr. Abdulmutallab was on a broad U.S. terrorist watch list, but he was not designated for special screening measures or placed on a no-fly list because of a dearth of specific information about his activities, Homeland Security Secretary Janet Napolitano said Sunday. She said he was screened properly before getting on the aircraft in Amsterdam. Mr. Abdulmutallab has claimed to law enforcement officials that he received training and instructions from al Qaeda operatives in Yemen.

The Saudi Arabia assassination attempt was carried out by a Saudi who was on the country's list of 85 most-wanted terrorists. The bomber was believed to have traveled to Yemen to connect with the al Qaeda franchise there. The bomber died in the explosion and is believed to have attached the explosives to his groin or inserted them inside himself.