SIOUX FALLS, S.D. (AP) — A North Dakota high school principal says appropriate action is being taken after three students briefly donned Ku Klux Klan-style white robes and hoods Friday night during a state hockey semifinal game.

The photo caused an uproar on Twitter when it was posted by 19-year-old Shane Schuster, who was seated with some friends at Ralph Engelstad Arena when something in the student section across the rink caught his eye.

“I thought, ‘Are those KKK hoods?’ I couldn’t believe it,” Schuster said. “I was shocked.”

Schuster said he focused his camera phone and snapped a photo, later uploading it to Twitter.

Kristopher Arason, Red River’s principal, said the school’s investigation determined that the students put on the attire just after Red River’s first goal and wore it for about 30 seconds to a minute. The teens removed the outfits after students in the section told them it was offensive, he said.

“We, as a school, are extremely disappointed with the behavior of these three students,” Arason said in a statement sent to The Associated Press on Saturday. “This behavior is not a representation of our school or student body.”

Arason said administrators were notified of the incident at the completion of the game. The students and their parents have been contacted and “appropriate action is being taken,” he said.

Arason did not indicate what disciplinary action, if any, the three unidentified students could face.

Red River topped Fargo’s Davies High School 2-0 to advance to Saturday night’s North Dakota Boys State Hockey Tournament title game against Grafton-Park River.

Davies High School is named in honor of Ronald Davies, the former federal judge from Fargo whose 1957 rulings integrated Central High School in Little Rock, Ark. — a pivotal event in the civil rights movement.

The photo that Schuster posted on the social media site shows the three hooded fans in the middle of the Red River Roughriders section, in which everyone is dressed in white as part of a “whiteout.” The post had been retweeted 75 times by late Saturday afternoon, with many users expressing their outrage.

The hockey tradition of encouraging fans to all wear all white was started more than 25 years ago by the original Winnipeg Jets — which currently are the Phoenix Coyotes. In 1987, Jets fans donning white shirts and jerseys packed Winnipeg Arena to watch the team take on the Calgary Flames in the first round of the Stanley Cup playoffs.

The practice has since spread to the college and high school levels.

Arason said Red River has a tradition of wearing a different color for each of the three days of the state tournament in accordance with the team’s colors. Roughrider fans wore black for the first day, white for the second and red for the final day.