WASHINGTON (CBS DC) — Department of Homeland Security investigators managed to sneak mock explosives and banned weapons through checkpoints in 95 percent of trials at Transportation Security Administration (TSA) checkpoints at the country’s busiest airports.

The internal investigation of TSA security checkpoints saw screening agents failing to detect the banned explosives and weapons in 67 out of 70 tests, ABC News reports. The DHS Inspector General went undercover in “Red Teams” to pose as passengers attempting to beat the system at some of the largest airports in the U.S.

A Department of Homeland Security Spokesman said such tests are a routine part of security “advancement.”

“Red Team testing of the aviation security network has been part of TSA’s mission advancement for 13 years,” explained the spokesman. “The numbers in these reports never look good out of context, but they are a critical element in the continual evolution of our aviation security.”

After learning of the high level of failure in the tests, Homeland Security Secretary Jeh Johnson “immediately directed TSA to implement a series of actions, several of which are now in place, to address the issues raised in the report.”

In one of the tests, an undercover agent was stopped after setting off an alarm at a magnetometer, but TSA agent screeners failed to detect a fake explosive device that was taped to his back during an ensuing pat-down, ABC News reports. Officials declined to divulge the exact timeline of when these tests were conducted.

And while Homeland security officials expressed frustration and disappointment at the test results, they insist that security at the country’s airports is still strong.

“These actions are in addition to a number of security enhancements the Secretary has directed TSA to implement to our aviation and airport security since the beginning of his tenure,” the spokesman added. “Today, all air travelers are subject to a robust security system that employs multiple layers of protection, both seen and unseen…In combination, these layers provide enhanced security creating a much stronger and protected transportation system for the traveling public.”

In 2013, a Red Team undercover investigator managed to get through a metal detector and a pat-down at New Jersey’s Newark Liberty Airport with a fake bomb hidden on his body.

“We know that the adversary innovates and we have to push ourselves to capacity in order to remain one step ahead,” a TSA official wrote on the agency’s blog in March 2013. “[O]ur testers often make these covert tests as difficult as possible.”

Explaining the test results at a 2013 Capitol Hill hearing, then-TSA administrator John Pistole described the Red Team as “super terrorists” who “know exactly what our protocols are” and how to expose such security gaps.