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As of Oct. 15, according to figures TSA provided to Congress, 1,121 facilities in 40 states have been certified to carry out prescreening of air cargo destined for passenger planes. The screening includes X-ray scanners like those used for checked baggage and, for some packages, physical inspections by staff or sniffer dogs, or the use of explosive-trace-detection technology.

But with foreign packages bound for the United States, inspection relies on foreign air-security or cargo-security systems, which often lack integrity, and where it is hard, or even impossible, to enforce U.S. standards.

“The threat doesn’t stop after screening,” said Mr. Mica. “You could screen these packages in Yemen and have that process compromised 40 times before it reaches its destination.”

Britain and Germany have suspended air-cargo deliveries from Yemen, Reuters reported, and Britain also has suspended them from Somalia. British officials said they were reviewing security on air cargo, but did not disclose any specific measures.

Britain also said it was banning air passengers from taking toner cartridges, like those used to conceal the Yemen bombs, onto planes as hand luggage, while Nigeria said it would improve its scanning of cargo bound for the United States.

According to the International Air Transport Association, 80 million metric tons of goods travel by air freight each year — most of it being electronics, engineering and machine parts, pharmaceuticals, and fruit and vegetables.

“There was stiff opposition [to the 2007 mandate] from industry, trade groups and some Republicans,” said Markey spokeswoman Giselle Barry.

The air-freight industry has argued that, because air-cargo packages are wrapped together with dozens of other items on large frames, or pallets, X-ray screening is often impossible without a huge investment of time and manpower that would create delays and raise costs.

“Who is going to pay for it?” said a former senior homeland security official who requested anonymity because of the sensitivities of the official’s current employer, adding that “the technology is not there” to do 100 percent screening without serious disruption to the flow of commerce.

“You need to strike a balance between practical security measures and the need to keep commerce flowing,” the former official said.

Norman Shanks, a former head of security at the British airports operator BAA, told Reuters that 100 percent screening would be unaffordable, but a tiered approach might be more practical.

“I think it will have to be affordable for packages of cargo coming out of countries that are known in the past to have caused problems. This will have to be a cost that is either picked up by the state itself or added onto the shipment,” he said.