Undercover agents were able to sneak fake explosives and banned weapons through Transportation Security Administration checkpoints, in an investigation that revealed a massive, system-wide security failure at America's airports. 

The findings were part of a DHS inspector general probe. The report is still classified, but Fox News has confirmed the investigation found security failures at dozens of airports. 

Homeland Security officials confirmed to Fox News that TSA screeners failed 67 out of 70 tests -- or 96 percent -- carried out by special investigators known as "red teams." 

In one case, an undercover agent with a fake bomb strapped to his back set off a magnetometer -- but the screener still failed to find it. 

Secretary of Homeland Security Jeh Johnson issued a statement late Monday outlining a series of actions he had ordered in response to the findings, including more training for all transportation security officers, re-evaluation of airport screening equipment and continued covert testing.

"We take these findings very seriously," he said.

The tests, and findings, were first reported by ABC News

The tests were designed to push the boundaries of the screening system -- set up after 9/11 -- to prevent weapons from getting onto planes. These tests are covert and designed to be as challenging as possible.   

Johnson reportedly sought a briefing on the findings last week at TSA headquarters. 

"Red Team testing of the aviation security network has been part of TSA's mission advancement for 13 years," the DHS spokesman told Fox News in a statement. "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." 

The spokesman said "several" of the changes Johnson directed following the report are now in place. 

"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 official said, stressing that all travelers are "subject to a robust security system that employs multiple layers of protection, both seen and unseen." 

On Capitol Hill, an aide to Senate homeland security committee Chairman Ron Johnson, R-Wis., said the chairman has not yet seen the full report but preliminary news reports "raise valid concerns in light of earlier inspector general findings and 'red team' tests of TSA's vulnerabilities." 

The aide, said, "Taxpayers have spent billions of dollars to secure our air travel system. It's important for that goal to be met." 

Fox News' Catherine Herridge and Matthew Dean contributed to this report.