The nation’s largest retailer has apologized after a photograph of a Walmart store seemingly advertising firearms as part of a back-to-school sale went viral this week.

The photograph, first tweeted by Twitter user Ismail Kidd Noorzai, shows a rack of rifles for sale on display inside an unidentified Walmart store beneath a sign reading: “Own the school year like a hero.”

Walmart described the image as “truly awful” and “horrible” through its own Twitter account and said it’s under investigation.

“What’s seen in this photograph would never be acceptable in our stores,” Walmart spokesperson Charles Crowson told The Washington Post. “We regret this situation and are looking into how it could have happened.”

Walmart initially identified the display as being inside a store in Evansville, Indiana, and tweeted that the sign “had no business there and was taken down as soon as we were alerted.” Mr. Crowson later said it erred, however, and was working to rightly identify the Walmart in question.

“We incorrectly identified the Evansville store,” he told Talking Points Memo. “We’ve corrected that and are now trying to find the accurate store.”

“Own the School Year Like a Hero” is an unrelated superhero-themed back-to-school promotion currently underway at Walmart stores nationwide.

Walmart sold firearms at upwards of 1,800 of its 4,000 U.S. stores as of 2013, likely making it one of the nation’s single largest licensed gun dealers, Time Magazine previously reported.