The discovery of explosive devices being shipped to the U.S. this week raised fears that al-Qaida was plotting new terror attacks that would exploit weaknesses in screening air cargo.
Airline passengers have been asked to remove shoes and laptop computers when they go through security, but the changes in handling of air cargo since 2001 have been much less visible.
The Transportation Security Administration is responsible for overseeing the safety of cargo shipments in the United States, but security measures differ from country to country.
Richard Bloom, an aviation-security expert at Embry-Riddle Aeronautical University, said because terrorists change their tactics, security guidelines need to be flexible.
"Any mandatory requirement can be known by the people who want to violate security," Bloom said.
Some questions and answers about cargo rules:
Q. Will the discovery of explosive materials in shipments bound for the U.S. lead to changes in cargo rules?
A. The U.S. Department of Homeland Security says it is taking extra steps to improve security including heightened cargo screening. The agency didn't give many details, and it said some of the changes won't be apparent to travelers.
Q. Do airlines in the U.S. have to inspect cargo shipped on passenger jets?
A. Yes. Since Aug. 1, airlines have been required to screen or inspect all cargo going on passenger planes flying within the United States or leaving the U.S. for other countries. A lot of cargo other than baggage gets shipped on passenger airlines. Cargo is a big revenue source for airlines.
Q. What about cargo coming to the U.S. on a passenger airline from another country?
A. That's a big weakness in the system. The Government Accountability Office estimated this summer that by August only 65 percent of all cargo coming into the United States would be screened. TSA has been talking to officials in other countries to win their cooperation in stricter screening of cargo headed to the U.S.
Brandon Fried, a cargo security expert and executive director of the Airforwarders Association, said most countries are eager to screen cargo.
"They want to protect their people, they want to protect their airplanes. They realize what's at stake here," he said. "The notion of them not doing it, or just putting cargo willy-nilly on a plane is just crazy."
Q. What about cargo-only airlines?
A. The 100-percent screening requirement for passenger airlines doesn't apply to cargo shipped on cargo-only airlines. Which cargo gets "positively screened" _ use of x-ray machines, swabbing packages for chemicals used in explosives, computer verification of the shipper's identity _ depends on the shipper and the security program.
After the Sept. 11, 2001 terror attacks, the government created a program called the Customs Trade Partnership Against Terrorism. Cargo airlines and companies that ship goods to the U.S. develop security plans. If the plans are approved by U.S. officials, cargo from those airlines and companies speeds through customs and is less likely to be screened.
Some cargo airlines go beyond government requirements depending upon the cargo, where it originates and if the shipper is well-known to the airline. Cargo from an unknown shipper in a part of the world where the drug-trade is particularly strong is more likely to be screened for drugs, for example.
Q. How is cargo screened?
A. The process is growing more automated all the time. Since the beginning of 2009, the TSA has approved dozens of new machines used to scan shipments for traces of explosive materials, chemicals and other substances. Screeners use everything from handheld wands to 12-foot-high X-ray machines to examine cargo.
Q. Who does the screening?
A. It can be done by the airlines themselves or by other companies _ shippers and freight forwarders _ who are certified by the TSA. Shipments screened before they go to the airport are tagged to ensure that they aren't opened before being loaded on a plane.
Lowy reported from Washington, Koenig from Dallas. AP Airlines Writer Joshua Freed in Minneapolis contributed to this report.