A large group of boys at a rural Wisconsin high school prom was photographed displaying what appears to be the Nazi salute, prompting international outrage and a local investigation.

In the photo, more than 50 boys from Baraboo, Wisconsin, pose on outdoor steps. The vast majority are shown hoisting their right arms stiff and straight into the air, fingers pointed out, as Nazis did in Adolf Hitler's Germany. One boy in the front row has his arms down but his right hand is forming the OK sign, a symbol adopted by the white power movement.

The photo was posted Sunday on Twitter by an account called @GoBaraboo, with the caption, "We even got the black kid to throw it up #BarabooProud." That account has since been made private.

Lori Mueller, the Baraboo School District superintendent, wrote to parents Monday, saying the photo was taken last spring. She said it was not taken on school property or at a school-sponsored event, but others in the community said the photos were associated with the school's junior prom.

"The school district is investigating this situation and is working with parents, staff and local authorities. If the gesture is what it appears to be, the district will pursue any and all available and appropriate actions, including legal, to address the issue," she wrote.

In a Twitter post Monday, Mueller added: "The photo of students posted to #BarabooProud is not reflective of the educational values and beliefs of the School District of Baraboo."

The police department in Baraboo, about 40 miles north of Madison, Wisconsin, said on Twitter it is aware of the photo and officers are assisting the school district with its investigation.

The incident is the latest in a string of inflammatory, white nationalist incidents across the country.

In Wisconsin, one parent called the Baraboo High School environment "stifling."

"There were a lot of Confederate flag tattoos and people throwing the n-word around, but there wasn't a super diverse population at the time," Mandy Lynn, whose daughter attended school in the district through 2012, said in an interview.

Another photo taken at the same spot featured boys and girls and was posted on Twitter in May by Jake Boll, a history teacher at the school. Boll's photo was taken from farther back than the professional photos and shows parents or other adults taking their own pictures of the group.

While the superintendent said the event was not school-sponsored, Boll advertised tickets to prom night on social media, with proceeds going to the junior class. In an email, Boll declined to comment and on Monday removed his photo from Twitter.

In the photo of the boys engaged in the apparent salute, one student, in the upper-right-hand corner, does not make the gesture. He identified himself as Jordan Blue to journalist Jules Suzdaltsev.

Jordan did not immediately respond to requests for comment. Reached by phone Monday morning, his stepmother, Melissa Blue, said she believed the photographer had suggested the pose.

"It absolutely is not in character for [Jordan] to do what those boys were doing, and he's visibly uncomfortable in the photo," Melissa Blue said. "From my understanding, it was something the photographer suggested and I don't think the boys really knew the impact. They're teenage boys."

The photographer did not respond Monday to an email sent through his photography website.

Melissa Blue confirmed the photo was from the school's junior prom in May.

"He's a good kid," she said of her stepson. "He's very good at standing for what he believes in."

As the picture spread Monday on Twitter, outrage was widely registered, including from the Auschwitz Memorial account. Auschwitz was the site of a Nazi concentration and extermination camp in occupied Poland.

"It is so hard to find words," the memorial's tweet said. "This is why every single day we work hard to educate. We need to explain what is the danger of hateful ideology rising. Auschwitz with its gas chambers was at the very end of the long process of normalizing and accommodating hatred."

The photo appears to have been posted originally on the website of a local photographer, whose business is called Wheel Memories. An archived version of the photographer's website shows the photo in question, along with about 50 other images in a section labeled "BHS Prom Pic's." On Monday, the page had been updated with an explanation of sorts and an apology.

"It is too bad that there are those in society who can and do take the time to be jerks; knowingly and willingly to be jerks! The internet can be a wonderful tool but for some there is an overwhelming urge to destroy," the site now says. "To anyone that was hurt I sincerely apologize. To those who have harmed them, we as society often ignore them I have chosen not to do that. YOU ARE JERKS! Grow up!"

The Washington Post's Amy B Wang contributed to this report.