Thieves have stolen some 20 Nazi artifacts, including an SS uniform and a boy’s Hitler Youth outfit, from the Deutsches Museum Nordschleswig in Sønderborg, Denmark. It is the latest in a disturbing and growing string of robberies at war museums across Europe targeting artifacts linked to the Nazi regime.

“Uniforms are sought-after and valuable trophies in collectors’ circles, which could be a possible motive for the burglary,” the museum said in a statement on Facebook. The stolen SS uniform is estimated to be worth 200,000 Danish kroner (about $31,400), according to Monopol.

“Museum manager Hauke Grella is shocked by the break-ins and asks for help in clearing the crime,” the museum continued. “He will consult with experts promptly about an additional security of the museum and the exhibition items.”

Nazi uniforms on display at the Deutsches Museum Nordschleswig in Sønderborg, Denmark. Photo courtesy of the Deutsches Museum Nordschleswig.

The thieves appear to have been specifically targeting German World War II artifacts—particularly the Waffen-SS, the military branch of Adolf Hitler’s SS—leaving objects linked to the Allied forces behind.

The break-in follows a wave of similar thefts in the Netherlands. In August, six men broke into the Eyewitness Museum in Beek, breaking down the front door and smashing the display cases before reportedly making off with artifacts worth €1.5 million ($1.75 million).

In October, burglars at the Oorlogsmuseum in Ossendrecht stole more than $1 million worth of historic artifacts, including 23 mannequins with SS uniforms and firearms, under cover of night.

At Jan de Jonge’s Oorlogsmuseum in Ossendrecht, Netherlands, mannequins dressed as Nazi soldiers during German occupation were stolen by thieves. Photo by Jan de Jonge.

“It’s very sad for me. I’m 77 years old and I collected all my life. It was fairly difficult to get together,” Jan de Jonge, who operates the Oorlogsmuseum out of his house, told CBC Radio. “After the war, everyone hated the Germans, so they threw away a lot of things. And that’s the reason it’s now so scarce.”

To protect their collections from thieves, some museums have begun removing objects from display.

“Yesterday, I took stuff from the Hitler Youth, and uniforms of the SS are also being removed,” Frans van Venrooij, director of the 1940–1945 War Museum in Loon op Zand, told the Guardian.

The Overloon War Museum, meanwhile, is returning a loan of the Book of the Dead from Auschwitz to the Netherlands Institute for War Documentation in Amsterdam ahead of schedule.

The string of burglaries comes on the heels of a recent rise in the market for Nazi memorabilia, which has gone hand-in-hand with an increase in forgeries of artifacts from the era. A planned 2019 exhibition in Argentina was found to be full of fakes.