PETN, short for pentaerythritol trinitrate, is a widely available - and easily detected - chemical explosive that has a long history of terrorist use.
Terrorist incidents in recent years involving PETN include:
* The suspect in a failed Christmas Day 2009 airliner bombing attempt. Umar Farouk Abdulmutallab was traveling from Amsterdam when he tried to destroy the plane carrying by injecting chemicals into a package of PETN explosive concealed in his underwear.
* The Saudi government said PETN was used in an assassination attempt on the country's counterterrorism operations chief in August, 2009.
* 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 widely used in the plastic explosives terrorists used to blow up airplanes in the 1970s and 1980s.
According to Encyclopedia Brittanica, PETN is a highly explosive, colorless organic compound, and is related to nitroglycerin. Introduced as an explosive after World War I, PETN is "valued for its shattering force and efficiency ... and is the least stable of the common military explosives but retains its properties in storage for longer periods than nitroglycerin or cellulose nitrate (nitrocellulose) does."
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PETN is the primary ingredient in detonating cords used for industrial explosions and can be collected by scraping the insides of the wire. James Crippin, a Colorado explosives expert, told CBS last year it's also used in military devices and found in blasting caps. PETN is the high explosive of choice because it is stable and safe to handle, but it requires a primary explosive to detonate it, he said.
Addressing the 2009 Christmas Day bombing use of PETN, Crippin and law enforcement officials told CBS 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 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.
This would not have applied to Thursday night's attempts, which involved cargo planes rather than passengers carrying the explosive.