The latest terrorism scare, involving a pair of explosive packages bound for Chicago from Yemen, has shed light on a new target for bombers—aircraft traveling to the U.S. whose cargo holds either have not been inspected, or if they have, by x-rays and bomb-sniffing dogs  that are not sensitive enough to root out certain types of explosives. Would-be aircraft bombers have proved successful at smuggling pentaerythritol tetranitrate (PETN) into aircraft cabins concealed in shoes and underwear, but fortunately have been unable to detonate this high-powered, military-grade explosive in flight. The latest plot placed one PETN bomb and detonator in a cargo plane and another in the cargo hold of a passenger plane, where they were less likely to be detected.

One of the bombs, which contained 400 grams of PETN, was found at the East Midlands Airport in England, having traveled there on board a United Parcel Service cargo plane. The other device, which contained 300 grams of PETN inside a Hewlett–Packard desktop printer, was found in a FedEx package in Dubai after having traveled there on a Qatar Airways passenger flight. Both bombs contained far more explosive material than the 80 grams of PETN that Umar Farouk Abdulmutallab managed to smuggle onto a Northwest Airlines flight in his underwear while en route from Amsterdam to Detroit on Christmas day last year. Abdulmutallab's plan failed when he was unable to detonate the explosives. In December 2001 Richard Reid tried to use PETN hidden in his shoe to blow up an American Airlines flight from Paris to Miami.

Since Abdulmutallab's attempt, efforts to secure airline cabins have been stepped up, but security surrounding cargo planes and cargo compartments in passenger airplanes has received much less attention, particularly outside the U.S. In August 2007 President George W. Bush signed a law requiring that by August 2010, all cargo transported on domestic flights and on passenger aircraft flying into the U.S. pass through security screening. Congressman Edward Markey (D–Mass.), who authored the 2007 legislation, says the Transportation Security Administration (TSA) was inspecting about 80 to 85 percent of international cargo bound for the U.S. on international passenger planes as of August 1, and that the TSA is not expected to reach 100 percent screening of that cargo for two more years.

Cargo planes are not covered by the 2007 law. Instead the TSA relies on international shippers to follow the agency's security requirements covering access to shipping facilities and cargo. All international U.S.-bound aircraft carrying cargo must also provide cargo manifest information to U.S. Customs and the Border Protection (CBP) prior to arriving from international destinations.

The TSA expanded its use of Explosives Trace Detection (ETD) technology at security checkpoints around the country to screen carry-on baggage and passengers for traces of explosives after the failed Christmas Day attack. When using an ETD device, officers swab a piece of luggage or passenger hands and then place the sample inside the ETD unit, which analyzes the content for the presence of potential explosive residue. The U.S. Department of Homeland Security has awarded $15 million in American Recovery and Reinvestment Act funding for 400 fixed ETD units. The federal government's fiscal year 2011 budget includes $39 million to purchase about 800 portable ETD machines. (There are about 150 international airports in the U.S.)

In addition to ETD devices, the TSA uses two types of imaging technology to screen the nearly two million passengers passing through U.S. airports daily—millimeter wave and backscatter. Millimeter wave technology bounces electromagnetic waves off the body to create a black-and-white, three-dimensional image. Backscatter technology projects low-level x-ray beams over the body to create a reflection of the body displayed on the monitor.

The October 29 incident indicates that more attention must be paid to screening shipped packages. Yet, even if all cargo were screened, explosives, including PETN are difficult to detect because they have a very low vapor pressure, which means very little of the explosive material gets into the air around the bomb where it can be detected. "Normally, whenever there is a solid material sitting on a surface, there is a certain concentration of that substance lingering in the air above it in a gas phase," says physicist Kurt Becker, Polytechnic Institute of New York University's associate provost for research and technology initiatives. "All explosives are notorious for having a very low vapor pressure at room temperature." Cheese, by comparison, has a very high vapor pressure at room temperature and is easy to detect through its aroma.

Most explosives are not instantaneously combustible and require a detonator. In the October 29 incident the packages bound for Chicago contained bombs hidden inside Hewlett–Packard desktop printer cartridges. The Dubai package also contained a closed electric circuit connected to a mobile phone SIM card hidden inside the printer.

"You need to look for some type of detonator, which is easier to find than the explosives themselves because most types of detonators have metal in them—a wire or a microchip, for example—that triggers a small spark or electrical signal," Becker says.

Becker serves as a consultant for Austria-based gas analysis instrument-maker Ionicon Analytik, GmbH, which makes proton transfer reaction–mass spectrometry (PTR–MS) technology that the company claims can distinguish substances having very similar molecular structures as well as correctly identify explosives, chemical warfare agents and substances that could be combined to create a bomb. Ionicon's goal is to have its technology deployed throughout the airport screening process, both for passengers and packages. "We have a proposal pending for the Department of Homeland Security that would further develop the technology," he adds.