According to a  new report on airport security, concerns are raised as shocking results of an undercover test reveal as many as 75 percent of fake bombs made it through security checkpoints at some of the country's busiest airports.

In the security lines at Chicago's O'Hare International Airport, undercover government investigators say they were able to sneak through with fake bombs and bomb parts 60 percent of the times they tried.

In Los Angeles, the rate was 75 percent.  In San Francisco, the screeners were better but were still fooled 20 percent of the time.

Critics like former Homeland Security General Clark Kent Ervin say there's no excuse for contraband making it past Transportation Security Administration screeners.  "It is absolutely not acceptable," Ervin said.  "All these many years after 9/11. All these many years after the creation of the Department of Homeland Security." 

Surprisingly, some passengers are more sympathetic.  "I can imagine sitting at one of those monitors and looking at image after image," said frequent flier David Casek.  "It has to be a little numbing."

In fact, the Transportation Security Administration (TSA) says the screening procedures are better now than during the year-long investigation that ended late last year.  Because bombs don't look the same anymore, TSA's tests are intentionally difficult for screeners to pass.

"So we expect a significant failure rate because if tests were easy, everybody would be passing," said TSA spokeswoman Ellen Howe.  "We want the tests to be hard.  We want the tests to replicate the real world scenarios."