A monument in Berlin to the plight of gays at the hands of the Nazis has been vandalized.

The memorial is a gray slab of concrete with a window through which viewers can watch a video of two men kissing. It is located in Berlin's Tiergarten park.

According to the AP, vandals painted over the window. Authorities told the outlet that they have not identified any suspects.

The memorial was installed in 2008 and has been vandalized in the past.

An estimated 50,000 gays were labeled criminals and outcasts by Germany's Nazi party after Hitler outlawed homosexuality in 1936, calling it “unnatural” behavior unbecoming of the Aryan “master race.” As many as 15,000 gays were tortured and died of hunger, disease, or abuse in Nazi concentration camps, where they were identified with a pink triangle.