Pentaerythritol Tetranitrate (PETN), the explosive found in two bombs hidden in printer cartridges that were being shipped via jets from Yemen to the United States, were used in past Qaeda-linked terrorism plots.
According to the New York Times, in 2001, PETN was found hidden in the shoes of Richard C Reid during an American Airlines flight.
Last Christmas, Umar Farouk Abdulmutallab had three ounces of PETN hidden in his underwear on a Northwest flight from Amsterdam to Detroit.
An assassination attempt in August 2009 on Saudi Arabia's intelligence chief, Prince Mohammed bin Nayef, also employed PETN.
Other terrorist groups have also used PETN, and the presence of the explosive itself does not decisively point to Al Qaeda.
"That's a very common explosive. There's no reason to think a lot of people didn't have access to do that," the NYT quoted Jimmie C. Oxley, a professor of chemistry at the University of Rhode Island, as saying.
PETN, a white powder that was introduced after World War I, belongs to the same chemical family as nitroglycerin.
It is about 70 percent more powerful than T.N.T., and is stable.
PETN generally does not explode when dropped or set on fire.
Usually, a strong shock wave from a blasting cap or an exploding wire detonator is needed to set it off.