It is alleged that the site was used as a mass grave and crime scene where Yugoslav Partisans killed 12,000 Croats by forcing them into the mine and covering them with concrete in 1945.
Before the mine was closed with concrete on 8–9 May 1945, partisans forced 12,000 Croats to enter it, including 2,000 women and children.
The next day, the partisans closed the mine, leaving them underground to die from choking, dehydration, or hunger.
Some local Slovenes were arrested and prosecuted for reportedly giving testimony about the Pečovnik and Matjaževa pits to international journalists.
[1] Roman Leljak, with group of archeologists and historians, made the first investigations and archaeological excavations of victims in the early 1990s.