WASHINGTON — When covert agents test how well airport security screeners find homemade bombs, they pack a detonator, timer and battery inside a cluttered toilet kit, stuff it into a suitcase and carry it through a checkpoint.

Agents also hide fake sheet explosives in briefcase linings. They stash watch timers inside hollowed-out books. They cram detonators in back braces and smear plastic explosives on shoelaces.

The Transportation Security Administration's special operations division devised the testing to raise the stakes for airport screeners and test whether they can spot bomb parts hidden as a terrorist might try to get them on an airplane, according to a classified TSA report obtained by USA TODAY.

The testing in the past year is far harder than it was before and shortly after the TSA took over airport security in 2002, agency spokeswoman Ellen Howe said. In earlier tests, covert agents would put a gun or a large assembled bomb in an otherwise-empty briefcase, she said.

Howe said the increased difficulty explains why screeners at Los Angeles and Chicago O'Hare airports failed to find more than 60% of fake explosives that TSA agents tried to get through checkpoints last year.

The failure rates — about 75% at Los Angeles and 60% at O'Hare — are higher than some tests of screeners a few years ago and equivalent to other previous tests.

"We want to have higher failure rates because it shows that we're raising the bar and the tests are harder," Howe said. Using a basketball analogy, she added, "You might score more points against a high-school team, but your skills are going to be improved if you're playing against an NBA team."