An American traveller boarded a flight from the US to Japan carrying a gun which was undetected in airport security check, according to an official statement.
The US' Transportation Security Administration (TSA) said in the statement on Saturday: "TSA has determined standard procedures were not followed and a passenger did in fact pass through a standard screening TSA checkpoint with a firearm at Hartsfield-Jackson Atlanta International Airport on the morning of January 3."
Delta Airlines also issued a statement to CNN, saying that "upon the customer's disclosure, the airline reported the incident to the TSA".
The security breach came two weeks into the government shutdown, during which TSA agents have been required to work but have not received paychecks.
CNN reported on January 4 -- a day after the breach -- that hundreds of TSA agents from at least four major airports had called in sick.
The TSA however, dismissed suggestions the government shutdown contributed to the security lapse and said a normal amount of staffers were working that day.
The Administration noted that it will "hold those responsible appropriately accountable."
This is not the first time TSA screeners have struggled to detect weapons.
In 2015, the acting administrator for the TSA was reassigned after a report found that airport screeners failed to detect explosives and weapons in nearly every test that an undercover team conducted at dozens of airports.