The discovery of US-bound mail bombs on cargo planes in Britain and Dubai reveals the vulnerability of air shipping, which is governed by a patchwork of inconsistent controls that make packages a potential threat even to passenger jets, experts said.
Most countries require parcels placed on passenger flights by international shipping companies to go through at least one security check. Methods include hand checks, sniffer dogs, X-ray machines and high-tech devices that can find traces of explosives on paper or cloth swabs.
But security protocols vary widely around the world. Experts cautioned that cargo, even when loaded onto passenger planes, is sometimes lightly inspected or completely unexamined, particularly when it comes from countries without well-developed aviation security systems.
The fact that at least two parcels containing explosives could be placed on cargo-only flights to England and Dubai, one in a FedEx shipment from Yemen, was a dramatic example of the risks posed by the system, but the dangers have been obvious for years, analysts said.
One particular vulnerability: trusted companies that regularly do business with freight shippers are allowed to ship parcels as "secure" cargo that is not automatically subjected to further checks.
Even where rules are tight on paper, enforcement can be lax. A US government team that visited cargo sites around the world last year found a wide range of glaring defects, said John Shingleton, managing director of Handy Shipping Guide, an industry information service.
"They walked into a warehouse where supposedly secure cargo was," he said, declining to say where the site was. "Generally security is high, but if you think it's perfect you're kidding yourself."
Cargo companies have long shipped on passengers airlines, for whom cargo provides extra income.
Mike Boyd, who heads an aviation industry consulting firm in Colorado, said cargo is often put onboard passenger flights at the last minute, similar to passengers flying on standby.
X-Ray machines are not an effective tool to screen bulk cargo because of the large size and number of the items that need to be inspected, said Philip Baum, editor of Aviation Security International, while more sophisticated technology, like gamma-ray machines, are extremely expensive.