The report looks only at those three airports, using them as case studies to understand how well the rest of the U.S. screening system is working to stop terrorists from carrying bombs through checkpoints.

The failure rates at Los Angeles and Chicago stunned security experts.

"That's a huge cause for concern," said Clark Kent Ervin, the Homeland Security Department's former inspector general. Screeners' inability to find bombs could encourage terrorists to try to bring them on airplanes, Ervin said, and points to the need for more screener training and more powerful checkpoint scanning machines.

In the past year, the TSA has adopted a more aggressive approach in its attempt to keep screeners attentive — the agency runs covert tests every day at every U.S. airport, TSA spokeswoman Ellen Howe said. Screeners who miss detonators, timers, batteries and blocks that resemble plastic explosives get remedial training.

The failure rates at Los Angeles and Chicago are "somewhat misleading" because they don't reflect screeners' improved ability to find bombs, Howe said.

TSA chief Kip Hawley, responding to previous reports about screeners missing hidden weapons, told a House hearing Tuesday that high failure rates stem from increasingly difficult covert tests that require screeners to find bomb parts the size of a pen cap. "We moved from testing of completely assembled bombs … to the small component parts," he said.

Terrorists bringing a homemade bomb on an airplane, or bringing on bomb parts and assembling them in the cabin, is the top threat against aviation. "Their focus is on using items easily available off grocery and hardware store shelves," Hawley said.

A report on covert tests in 2002 found screeners failed to find fake bombs, dynamite and guns 24% of the time. The TSA ran those tests shortly after it took over checkpoint screening from security companies.

Tests earlier in 2002 showed screeners missing 60% of fake bombs. In the late 1990s, tests showed that screeners missed about 40% of fake bombs, according to a separate report by the Government Accountability Office, the investigative arm of Congress.

The recent TSA report says San Francisco screeners face constant covert tests and are "more suspicious."

Copyright 2007 USA Today