Last year, covert investigators -- known as red teams -- descended on two of the nation's busiest airports to see if government screeners could stop them from smuggling bombs onboard commercial planes.
The result was failure across the board, with screeners unable to detect simulated bombs and bomb components.
Seventy-five percent of the devices got by screeners at Los Angeles International Airport, while screeners at Chicago's O'Hare International Airport missed fake explosives 60 percent of the time.
"Flunking a test 70 or 60 percent of the time … is something that is so scary that the American people should be outraged that six years [after 9/11] this is still possible," House Homeland Security Committee member Rep. Ed Markey, D-Mass., told ABC News.
Department of Homeland Security officials say the low scores are the result of tests intentionally made much more difficult.
"I don't think we're ever going to get to the point that there's an A+ or perfect score, because if we ever get a perfect score it means the testers aren't pushing hard enough," Homeland Security Secretary Michael Chertoff said Thursday.
Screeners are being challenged in part because terrorists are getting increasingly creative and deceptive, using common household products to make bombs that can get past screeners.
In recent weeks, the government warned that terrorists might try to smuggle remote control toys in carry-on luggage to detonate bombs.
Markey, who represents a Boston-area district, added, "[9/11 hijacker] Mohammed Atta and the other nine who came to [Boston] Logan Airport in 2001 found the route of least resistance to bring their little knives and their little box cutters into the passenger cabins of planes. Al Qaeda is always looking for the easiest way to bring down a plane."
At Reagan Washington National Airport just outside the nation's capital, the Transportation Security Administration demonstrated the degree of difficulty of the new tests Thursday.
TSA displayed devices that testers used to try to get by security, some of which looked like sticks of TNT attached to a simulated bomb. Now, says agency spokeswoman Ellen Howe, testers use "very small, intricate and common items that can easily be concealed" in clothing or luggage.
But screeners employed by private companies at San Francisco International Airport fared much better than government screeners when they were put to the new tests, missing only 20 percent of the simulated explosives.
"They do a lot of on the job training," said airport spokesman Michael McCarron. "Daily, they do testing of the equipment and the personnel as they go through here."
Markey said more training and more screeners are needed at airports around the United States.
"The Bush administration has written a blank check to fight the war in Iraq," he said. "It's time now for them to spend the money so that we protect Americans here at home, where al Qaeda wants to attack us once again."
Homeland Security officials say tougher tests are now being conducted daily at airports across the country. They believe the government screeners will get increasingly better at detecting bombs over time, but it remains to be seen if they can improve fast enough.