Thailand has come under fire this week after photos of a superhero banner depicting Adolf Hitler went viral, causing backlash.

Students at Chulalongkorn University painted Hitler alongside Superman, Captain America, and Batman on a banner outside the art school in a tribute to this year’s graduating class. School officials said it was painted by “ignorant students” who didn’t realize the artwork would offend anyone.

“[We] would like to formally express our sincere apology for our students’ ‘Superhero’ mural,” art school dean Suppakorn Disatapundhu said in a statement. “I can assure you we are taking this matter very seriously.”

The students said the concept of the poster was to show that both good and bad exist in the world, which is why Hitler was done in shades of black and white rather than in vibrant color, like the rest of the subjects.

“They told me the concept was to paint a picture of superheroes who protect the world,” the dean said. “Hitler was supposed to serve as a conceptual paradox to the superheroes,” he said. “This kind of thoughtless display will not happen again.”

The banner has been taken down after being on display for two days.

Superman. Batman. Captain America. Iron Man. The Incredible Hulk. Hitler? #thailand #highered http://t.co/JImp0rI1CZ

— Dylan Kissane (@dylankissane) July 15, 2013

Image: Simon Wiesenthal Center