Updated

While the nation has turned its attention to securing the American railways and transit systems in the wake of the London terror attacks on Thursday, persistent vulnerabilities threaten other U.S. transportation systems, say lawmakers.

Though it's been nearly four years since the Sept. 11, 2001, terror attacks — an assault that used airplanes as missiles and prompted unprecedented screening for airport passengers — comparable screening of cargo that flies on commercial airlines remains elusive.

"It is unacceptable that freight stored in the cargo bay beneath passengers' feet is almost never inspected for bombs that could bring down the plane," Rep. Ed Markey (search), D-Mass., a member of the House Homeland Security Committee, said last month.

On Thursday, Markey said the London attacks should embolden U.S. lawmakers to get serious about plugging the holes in security — in all transportation areas.

"We must honor the memory of those who have been killed by our vigilance to protect democracy and prevent future attacks," he said in a statement.

Markey noted that Americans take public transportation 32 million times a day — 16 times more than they travel on domestic airlines. Nonetheless, since Sept. 11, 2001, Markey has been vigilant in pursuing efforts to close the "security loophole" that prevents cargo on passenger planes from being uniformly screened for explosives.

Markey and Rep. Chris Shays (search), R-Conn., had introduced amendments to the $34.2 billion 2006 Department of Homeland Security appropriations bill that would have assured screening of all cargo carried on passenger planes by 2008, and, would have notified passengers before that time that not all cargo is being inspected for explosives.

The amendments failed on May 17, just before the bill passed the full House, 424-1.

Markey and Shays, also a member of the House Homeland Security Committee, managed to pass an amendment in 2003 requiring 100 percent cargo inspections, but the Senate dropped it from its version of the DHS bill. The same amendment was again defeated in 2004.

Frustrated for the third year in a row, Markey lashed out at the Republican leadership in the House.

"Republicans are fighting to leave every homeland security loophole behind while putting the special interests first," he said.

Critics of the measure say not only is the technology not available to accommodate all of the different sizes, shapes and contents of air cargo, but screening 100 percent would grind commercial cargo transport to a halt.

"Manufacturing, food and other industries depend on cargo forwarders for rapid and efficient transportations. Screening every single piece of cargo would effectively eliminate the possibility of just-in-time transportation and cargo on passenger planes," said David Wirsing, executive director of the Virginia-based Airforwarders Association, which represents the air cargo industry.

"The Markey-Shays solution would bring cargo transportation to a virtual standstill, resulting in higher costs for America's companies and consumers," Wirsing added.

Shays, who has support from survivors of the Sept. 11 attacks, said Americans should at least be made aware that while their pocketbooks, shoes and luggage are routinely screened for explosives, the cargo that goes in air with them is not.

"I think we owe it to all families who have lost loved ones in acts of terrorism to make our airlines as safe as we can," he said, noting support from Voices of September 11th, the Association of Flight Attendants and the Coalition of Airline Pilots Association.

"If we cannot screen cargo, then passengers deserve to know their safety is being compromised," Shays added.

Officials with the Transportation Security Administration, a division within DHS, said they are working daily on a solution since the level of technology currently available to accomplish 100 percent screening does not exist. They say a solution that would satisfy Markey and Shays is still out of reach.

"We recognize there is no perfect solution at this time that will address all potential threats to the air cargo supply chain, and that it will take time to develop and fully deploy a robust freight assessment system," said TSA spokeswoman Yolanda Clark.

Clark said recent testing of existing technology underscored flaws in several areas — machines need to be bigger and capable of handling large containers full of cargo of varying consistencies, for instance.

The machines that currently operate in large hangars are susceptible to environmental exposure and breakdowns. But the TSA is taking an array of measures to fill in the gaps, she said.

Bomb sniffing dogs routinely check cargo while smaller cargo are put through the same explosive detection devices as passenger luggage. In the meantime, the "known shipper" program requires that the shipper have a dated history with the airline in order to ship on passenger planes. If not, their cargo is placed on all-cargo transport, which has less stringent standards, but still must comply with TSA guidelines.

The bigger shippers, like Fed-Ex, have also been engaging in their own stringent "known shipper" programs.

"TSA is implementing a layered, air cargo security solution," said Clark.

The Markey-Shays team disputes the government's claim the technology isn't there yet, pointing to a number of companies, including California-based Rapiscan Systems, which says it's currently installing its machines as part of a TSA program in two airports in Texas and Alaska.

The Rapiscan models claim to deploy "Pulsed-Fast Neutron Analysis," which would allow rapid screening of bulk, containerized cargo at six to 10 containers per hour.

"While cost and level of risk should factor into this debate, the question of the availability of the technology has already been answered," said Peter Kant, vice president of government affairs for Rapiscan, in a letter to Markey in May.

Meanwhile, Markey and Shays have hedged their bets by introducing a stand-alone bill that would require the total cargo inspections and the passenger notification contained in their amendments. The Air Cargo Security Act, introduced May 3, was referred to a Homeland Security subcommittee.

Rep. Peter King, R-N.Y., a member of the Homeland Security Committee who did not vote for his colleagues' doomed amendments, said he acknowledges the loopholes and looks forward to further hearings on the issue.

"It's definitely a concern," he said. But, "it's expensive and you're talking about a large undertaking. It's a question of how we should do it and when we should do it."