When undercover agents posing as airline passengers hid fake bombs in their carry-on luggage or under their clothes, security screeners at two busy U.S. airports failed to detect most of them, according to a shocking classified report.

The report, obtained by USA Today, reveals that screeners at Los Angeles International Airport missed about 75 percent of simulated explosives and bomb parts.

Screeners also missed about 60 percent of hidden bomb materials at Chicago O’Hare International Airport.

“That’s a huge cause for concern,” Clark Kent Ervin, the Homeland Security Department’s former inspector general, told USA Today, adding that the results point to the need for more screener training and more effective checkpoint scanning machines.