The Transportation Security Administration said it failed to detect a box cutter used to menace passengers on a flight last week, officials said Monday.

The Frontier Airlines flight out of Cincinnati was heading for Tampa, but made an emergency landing in Atlanta Friday after the unhinged traveler began waving the blade.

The TSA said the suspect — who had a temporary Ohio driver’s license — passed through a security checkpoint with two backpacks.

Scanners failed to detect any weapons, but the man was tapped for an additional search.

A security agent found a boxcutter in one of the backpacks, but allowed him to keep it after removing the blades.

A second boxcutter went completely undetected in his other bag, the agency acknowledged.

“The backpack containing the other box cutter, and the remainder of the traveler’s property, was screened for explosives, but the box cutter was not discovered,” the agency said, adding that several employees would receive additional training because of the lapse.

“Nationally, TSA will issue a shift brief for all screening employees on this incident with reminders on use of the technology tools and prohibited items,” the agency added.

A frightened passenger told a flight attendant on the flight that the suspect was threatening his neighbors with a box cutter and said he was ready to slash them.

Passenger Larry Cumberbatch, a veteran, told ABC News that he and two others, a law enforcement officer and another military man, helped to restrain the suspect.

“You could see the energy ramping up where uncertainty, the uncertainty of what’s going to happen with this young man,” Cumberbatch told the outlet.

After the suspect made an abrupt move, the unnamed former cop tackled and subdued him before he was taken into custody in Atlanta.

No one was harmed during the incident, and passengers were put up in a hotel for the night before making alternate travel plans.