Dutch police said Friday they are investigating a stunt that saw a text alluding to an antisemitic conspiracy theory projected onto the Anne Frank House museum in Amsterdam, causing outrage across the country.

The words “Ann (sic) Frank invented the ballpoint pen,” referring to a debunked claim that the Jewish teenager’s famed diary is a forgery, were displayed for several minutes this week on the side of the building where her family hid during the Holocaust.

The 17th-century canal house is now a museum focusing on Frank’s short life, which receives around 1 million visitors a year.

Dutch Prime Minister Mark Rutte condemned the “reprehensible” incident, and tweeted: “We can never and should never accept this.” Justice Minister Dilan Yesilgöz-Zegerius cited the incident to urge parliament to approve a pending bill explicitly banning Holocaust denial.

The proposed legislation would make it easier to prosecute over the Amsterdam incident, which currently falls under a law on discriminatory statements against minority groups.

Amsterdam police said they are looking into the incident.

“We were notified about it and our detectives are investigating,” spokesperson Rob van der Veen told the AP.

The text was projected from a vehicle across the canal and was noticed by security guards, who contacted police. A recording of the stunt was posted on an antisemitic Telegram channel.

Frank kept a diary of life under German occupation in World War II, when, as a Jew, she was in constant danger. Even though she was arrested with her family in 1944 and sent to a Nazi concentration camp, where she died, the diary survived and became one of the world’s most famous books.

According to the Netherlands’ top official for fighting antisemitism, Eddo Verdoner, several pages written with a ballpoint pen were found amongst Frank’s papers in the 1980s. That type of pen was not introduced in the Netherlands until after WWII, and Holocaust deniers have claimed this proves her diary, published by her father after the war, is a fake. However, researchers have concluded that the pages were accidentally left in the diary in the 1960s.

Verdoner told the AP that the Amsterdam stunt was “a despicable act that tries to cast doubts on the experiences of the witnesses of the Holocaust.”

He said there had been a rise in antisemitism since the COVID-19 pandemic, noting that throughout history Jews have faced increased hatred during times of hardship.

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