Transportation Security Administration screeners at Los Angeles International and Chicago O'Hare failed to find fake bombs on undercover agents posing as passenger during more than 60 percent of tests last year.
At LAX, the screeners failed to stop passengers with bomb materials and bomb parts in 75 percent of 70 tests.
At O'Hare, the screeners performed only slightly better, catching the "bad guys" only 40 percent of the time during 75 tests.
"That's a huge cause for concern," Clark Kent Ervin, the Homeland Security Department's former inspector general, told USA Today.
In the past year, the TSA has adopted an aggressive program to make screeners more attentive. The agency runs covert tests every day at every U.S. airport.
Screeners who miss detonators, timers, batteries, and simulated plastic explosives blocks are sent to remedial training.