A lack of systematic screening requirements for cargo planes creates a large and easily exploited loophole for terrorists, experts said after international authorities discovered two explosive devices on UPS planes bound for the United States.
"There is no requirement for systematic screening of cargo sent on cargo planes, only on passenger planes," said Stephen E. Flynn, president of the Center for National Policy and a member of the government's bipartisan National Security Preparedness Group.
"We should not be surprised that the bad guys have figured that out and have selected cargo planes for sending explosive materials to the U.S.," he said.
Suspicious packages from Yemen intercepted in Dubai and England on Friday tested positive for explosives.
In an address Friday from the White House, President Obama called the discovery "a credible terrorist threat against our country."
Law enforcement authorities long have suspected the cargo network could be exploited by terrorists. While much has been done in the way of enhancing security at passenger airports and shipping ports, air freight depots remain vulnerable, said experts and lawmakers.
In a 2007 Congressional report, the air cargo system was called out for its susceptibility to an explosives plot.
"The air cargo system is a complex, multi-faceted network that handles a vast amount of freight, packages and mail carried aboard passenger and all-cargo aircraft," read the report by the Congressional Research Service. "The air cargo system is vulnerable to several security threats including potential plots to place explosives aboard aircraft [and] illegal shipments of hazardous materials."
The device found in the U.K. was a bomb disguised in a printer toner cartridge and laden with PETN. That's the same explosive carried by 2009's foiled Christmas day bomber. And like that plot, these bombs originated in Yemen.
British authorities only discovered the bomb after receiving quality intelligence from Saudi Arabia and not as a result of screening measures, authorities said.
Officials say packages are screened at American depots using x-ray scanners and sniffer dogs, but the largest threat to the U.S. comes from foreign countries that do not have stringent security measures.
Sen. Chuck Schumer, D-N.Y, said the Transportation Security Administration is developing new technology to screen packages but such programs are not being implemented fast enough.
"It has to be technology," Schumer told ABC News affiliate WABC. "You can develop screening devices. The first priority for TSA was for passenger devices. We're about to embark on better screening for luggage. ... As for big containers, that's third on the list. It should be done more quickly. I've been pushing them. ... This is a wake-up call."
But despite insisting that technology could sniff out future explosives, Schumer said it was human intelligence that led to the bombs being discovered.
"The good news is we're able to listen in on what terrorists do much more effectively than ever before," he said. "There was word about packages in Dubai and London and we wanted to be super careful. ... This won't be over until we're certain these were the only packages sent."
The Department of Homeland Security issued a statement that it has implemented new measures to enhance security at airports.