And experts caution that cargo, even when loaded onto passenger planes, is sometimes lightly inspected or even completely unexamined, particularly when it comes from countries without well-developed aviation security systems.
About 60 percent of all cargo flown into the U.S. is on passenger planes, according to Brandon Fried, a cargo security expert and executive director of the Airforwarders Association. New jumbo jets flying in from overseas - like the Boeing 777 - have "cavernous" bellies where freight is stored, he 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 air shipping is governed by a patchwork of inconsistent controls that make packages a potential threat even to passenger jets, experts said Saturday. Security protocols vary widely around the world, whether they're related to passenger aircraft or cargo planes.
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, but the dangers have been obvious for years, analysts said.
Some Western countries, perhaps belatedly, are trying now to manage the risks.
Britain's Home Secretary Theresa May said the device discovered early Friday morning at England's East Midlands Airport was potentially able to explode - and could have been used to bring down a plane. She said the U.K. has now banned the movement of all unaccompanied air freight originating from Yemen.
France's civil aviation authority also suspended air freight from Yemen, as did the world's largest package delivery companies - FedEx and UPS.
One particular vulnerability in the system: 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.
And even where rules are tight on paper, enforcement can be lax. A U.S. government team that visited cargo sites around the world last year found a 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 that was. "Generally security is high, but if you think it's perfect you're kidding yourself."
Cargo companies have long shipped on passenger airlines, for whom cargo provides extra income. Passenger planes often carry the most perishable goods shipped internationally, like live seafood, fresh flowers and even human organs.
Authorities are well aware of the risks cargo aboard passenger planes poses, as shown by the decision Friday to have American fighter jets escort an Emirates Airlines passenger jet down the coast to New York to keep track of it until its cargo had been inspected.
Air freight generally consists of the most expensive cargo, everything from designer clothing and prescription drugs to car parts and mobile phones. Freight is often transported in large pallets, which are generally not taken apart to inspect because the process would significantly slow down air travel and the movement of goods.
About 50,000 tons of cargo is shipped by air within the U.S. every day, according to the Transportation Security Administration. About 25 percent of that is shipped by passenger airlines. 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.
Inside the U.S., new rules that took effect in August require that every piece of cargo be checked for explosives. Cargo is increasingly screened by X-ray machines and handheld wands - the TSA has approved dozens of new machines in the past two years that can detect traces of explosive materials.
But those rules don't cover goods coming in from other countries.
"We've known for decades that freight isn't as secure - this isn't a surprise," security expert Bruce Schneier said. "You can't protect every package. There's no way."
Cargo that travels through airports in countries with high threat levels and advanced security systems is often safer. The system at London's busy Heathrow Airport is relatively effective because cargo is held for 24 hours, giving authorities time to check it properly, according to Shingleton.
Still, since August U.S. aviation officials have been pressing the European Union to require the X-raying of every package placed on passenger planes, but they have met resistance because of the cost and logistics involved in screening such a huge amount of material, aviation safety consultant Chris Yates said.
"Is it possible one of these devices could get on passenger jets?" Yates said. "I'm not convinced it could on flights between London and the States, but it could get on from less secure parts of the world, including the Middle East. If you talk to anybody senior at airports, they will tell you freight is the weak link in the chain."
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.
Baum warned that it would be foolhardy to downplay the threat posed by cargo-only planes, since those could be loaded with an explosive device that could be detonated when the plane is on its final approach over a major city.
"Security in the UK is pretty good, the U.S. is not bad, but aviation is a global business and we need effective regimes around the globe," he said. "Cargo travels on both cargo-only and on combi-aircraft, which have passengers and cargo, and cargo is not subject to the same screening requirements as passengers' baggage."
Bomkamp reported from New York. Raphael G. Satter in London and Adam Schreck in Dubai contributed to this report.