Darren Staples/Reuters
A UPS cargo plane is seen near an advertisement at East Midlands Airport in Castle Donington, central England, on Nov. 1. Governments, airlines and aviation authorities around the world were reviewing security on Monday after US-bound parcel bombs sent by air from Yemen were intercepted in Dubai and Britain.

PETN: How serious a threat is Yemen explosive to air travel?

American X-ray machines should spot PETN, the explosive used in the suspicious packages from Yemen, in any luggage. But cargo flights from abroad could be vulnerable.

Just the concept alone – someone shipping powerful plastic explosives on an airplane – is hair-raising. That’s what happened last week when two bombs sent from Yemen were intercepted in Britain and Dubai.

They got through security because they had been crammed into printer ink cartridges and then disguised as normal packages.

Yet the latest terrorists’ explosive of choice – pentaerythritol tetranitrate (PETN) – can be detected, says a leading US expert on explosives. And international airline passengers should not be alarmed because airport security is aware of PETN’s characteristics, which means it is not likely to make it past scanners.

However, cargo flights from abroad could be vulnerable. This is one reason that some countries on Monday suspended air freight shipments from Yemen.

American X-ray machines would spot the explosive if it were part of any luggage, says Jimmie Oxley, a professor of chemistry at the University of Rhode Island in Kingston and a leading expert on explosives.

“It has a distinctive characteristic of high explosives in that it is more dense than normal materials,” Dr. Oxley says. “If your luggage gets swabbed, it can also be detected.”

Congress, she says, first named PETN as an explosive that should be detected as far back as 1988. “We’ve had a long time to work on it,” she says, adding, “There is lots of room for improvement, and we’re working on better ways to get the job done.”

The intercepted bombs appear to be quite sophisticated, bomb experts say. “They appear to be put together by someone who knows how to construct powerful bombs,” says Lawrence Kobilinsky, a professor of forensic science at John Jay College in New York.

According to news reports, the US suspects Saudi-born Ibrahim Hassan al-Asiri of building both bombs that have been intercepted. Mr. Asiri is thought to be hiding in Yemen.

The bombs appear to be made so they could be detonated by a remote signal, such as via a cellphone, Mr. Kobilinsky says. “That makes them even more sophisticated,” he says.

Although most US aircraft would be vulnerable to such a signal, the Israelis have developed a method to block remote detonations via cellphones, he says.

“Maybe it’s the next step for planes,” says Kobilinsky.

Controlling air freight shipments may be somewhat easier for carriers such as FedEx and United Parcel Service to deal with. That’s because they’re familiar with many of their customers.

“If it’s a lifelong customer who brought in the same stuff every day and shipped it to the same place, your threshold for pain is pretty high,” says Dan Gallington, a senior fellow at the Potomac Institute for Policy Studies in Arlington, Va. “But if it’s someone who walks in off the street, you have a different protocol.”

The shippers have the right to open up suspicious packages, he says. “Once you have the item to be shipped, you can do any kind of intrusive inspection, unwrap it, and look at it,” says Mr. Gallington, who formerly served in the US Justice Department in security areas.

Yet Gallington believes that Congress should hold hearings to determine what, if anything, should be done to get air freight companies to further tighten their security.

“Congress needs to make sure it’s not falling into some regulatory abyss,” says Gallington, who says it’s not clear to him which federal agency has jurisdiction.

“I bet FedEx and UPS wrote their own rule book on how they do business,” he says.

Also, he says he is not comforted by current efforts to identify plastic explosives. For example, he says, the so-called Detroit underwear bomber used thin layers of plastic explosives. He worries someone will produce an even thinner version that avoids detection.

“Could I make a suit out of this stuff and walk through sensors and then make a bomb out of it?” he asks.

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