BERLIN – The head of the Buchenwald Memorial site on Thursday blamed growing right-wing populism in Europe for an incident in which members of a British neo-Nazi group appear to have performed a “Hitler salute” at the former concentration camp.

German daily Bild reported that a picture posted on a social media site linked to the group National Action showed a blurry image of two people holding the far-right group’s flag, their right arms raised in salute. The photo was posted on the group’s Twitter page.

The Buchenwald Memorial Foundation said it filed a criminal complaint to police after being informed of the picture.

Volkhard Knigge, the foundation’s director, said it recorded 20 incidents last year that were attributed to German far-right extremists. The figure was twice as high as in 2014, he said.

“In view of the growing right-wing populism in other European countries provocations by foreign neo-Nazis were also feared,” said Knigge. “The photo apparently staged by British neo-Nazis in the basement of the crematorium of Buchenwald concentration camp demonstrates a specific criminal, ideological energy.”

“This is a serious denigration of the almost 280,000 prisoners at Buchenwald, among the Britons,” said Knigge.

The room in which the picture was allegedly taken was used by the Nazis to store the corpses of the estimated 56,000 people killed at Buchenwald before they were burned. The SS also strangled more than 1,000 men, women and children on metal hooks attached to the room’s walls.

Knigge said the room was guarded, meaning that the two people pictured on the photo must have planned their stunt in advance.

Buchenwald, near the eastern city of Weimar, was opened in July 1937. U.S. forces liberated the camp on April 11, 1945.