"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," the release states.
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 first reported on January 4 -- a day after the breach -- that hundreds of TSA agents from at least four major airports had called in sick.
However, the TSA dismissed suggestions the government shutdown contributed to the security lapse and said a normal amount of staffers were working that day.
"The perception that this might have occurred as a result of the partial government shutdown would be false," TSA said. "The national unscheduled absence rate of TSA staff on Thursday, January 3, 2019, was 4.8% compared to 6.3% last year, Thursday, January 4, 2018. So in fact, the national call out rate was higher a year ago than this year on that date."
The TSA noted that it will "hold those responsible appropriately accountable."
"Security standards will NOT and have NOT been compromised," said Michael Bilello, TSA assistant administrator for public affairs, on Twitter.
"While I realize this is not what you are owed for your hard work ... and what you deserve, I hope these actions alleviate some of the financial hardship many of you are facing," TSA Administrator David Pekoske wrote.