It was 2 a.m. on a Tuesday when the raid began at the Eyewitness War Museum in the Dutch town of Beek.

First, a group of thieves teased the front gate of the museum. “You can see it on our cameras,” said Wim Seelen, the museum’s director, in a telephone interview.

But then they disappeared.

An hour later, the burglars returned in several station wagons. In a scene reminiscent of a robbery movie, they put tires on the highway that passes the museum to create a roadblock and parked a fake police car next to it to make it look official.

In the next five minutes, the group – maybe 12 people in total, said Seelen – smashed in the front door of the museum, broke showcases and took what they wanted: nine mannequins in rare Nazi uniforms. The outfits included one from Hitler’s personal cook and one from a high-ranking SS member

The robbers took away other items from World War II memorabilia, Seelen said, valued at about $ 1.5 million in total.

“It was done with military precision,” he added. The museum’s alarm went off, but the police – held up by the roadblock – were too late to catch anyone.

“Of course I’m afraid it will happen again,” said Seelen.

The August 4 raid in Beek was only the most dramatic in a string of World War II museum robberies in Europe, and the break-ins spread panic among similar institutions.

Since March, four museums in the Netherlands and Denmark have been broken into and memorabilia, including Nazi uniforms, stolen. The last raid took place on November 3, when robbers broke through a window in the Deutsches Museum Nordschleswig in southern Denmark and ran away with three mannequins in Nazi outfits.

Administrators of all four broken-in institutions said in telephone interviews that they believed the thieves were acting on the orders of collectors to get their hands on rare Nazi memorabilia. However, they weren’t sure if the robberies were being carried out by the same group or were simply part of a worrying trend.

Dutch and Danish police officers said in telephone interviews that they did not have any suspects in any of the robberies but were looking for patterns.

Richard Bronswijk of the Dutch Police Department’s Art Crime Department said his team had two theories: that wealthy collectors in Russia or Eastern Europe ordered the robberies or that they were committed by the far right. The second theory is less likely, he added, “because these people don’t have a lot of money and like to buy replicas.”

The raid on the Eyewitness War Museum was incredibly professional, he said. “They were really like ‘Ocean’s Twelve,'” he added, referring to the Hollywood heist movie.

In the Netherlands and Denmark, both occupied by Nazi Germany during World War II, there are numerous small, private and government-funded museums devoted to the history of this conflict. Many have glass cabinets with memorabilia such as weapons and dioramas depicting scenes from the war with mannequins in original uniforms. Seelen estimates there are around 100 in the Netherlands alone.

Many Dutch museums have removed rare items from the exhibition or upgraded their security systems in response to recent robberies. The Arnhem War Museum has installed anti-tank barriers at the entrance “so that people can’t come with a big truck,” said Marina Moens, one of its owners.

Concern is growing in Denmark too. “I’m sure every museum takes precautions,” said Henrik Skov Kristensen, director of the Froslev Camp Museum, in a telephone interview. “But if someone is determined to do something like that, they will.”

Mr Kristensen’s museum in a former prison camp in Denmark was robbed in March. The burglars also took away SS uniforms. After the Danish police found no evidence, they closed the investigation in April.

Giel van Wassenhove, a Belgian military memorabilia dealer, said in a telephone interview that the value of Nazi items had increased for years. “The stuff that is stolen is all very desirable, and the prices are going insane,” he said. “Everyone knows if it has a Nazi badge on it, the price is high.”

An SS uniform could fetch between $ 3,500 and $ 35,000, he said.

During the two Dutch robberies, thieves stole a special rifle, the “FG 42”, which was used by Nazi paratroopers, said van Wassenhove. A decade ago, he said, that weapon was worth about $ 60,000; Today it’s worth more than $ 175,000.

But Mr van Wassenhove downplayed the suggestions that a boom in right-wing collectors was driving the rising prices. Most of the buyers were investors who were simply pursuing a profit, he said.

Many museums may not see the value of objects in their collections, added van Wassenhove.

Those who do are taking no chances: Ms. Moens of the Arnhem War Museum said that the museum not only installed anti-tank barriers, but also displayed all of its Nazi uniforms. In October, the Overloon War Museum returned two rare books it had borrowed from an Amsterdam institute, including a “Book of the Dead” listing 1,500 Holocaust victims in Auschwitz. Janneke Kennis, a spokeswoman for the museum, said the museum feared the books could be attacked by thieves.

Mr. Seelen said the raid on the eyewitness museum was so devastating that he considered closing it for weeks. He said he knew he would never see the items again.

World War II museums, however, are not just homes for memorabilia; they play an important educational role.

“World War II was a time of so great suffering that we have to tell the story of it to make sure it never happens again,” he said. “I’m not going to stop telling this story.”