A nonprofit organization that runs the Women’s Active Museum on War and Peace received a bomb threat earlier this month, it said on its website Sunday.

The museum, in Shinjuku Ward, displays material on the “comfort women,” Japan’s euphemism for those who were forced to work in its military brothels during the war.

The group said the threat came in a letter from one “Asahi Sekihotai” that arrived Oct. 5 and demanded the NGO remove its war-related displays. It was postmarked Sept. 30 in Shinjuku Ward.

In a news release, the NGO said it reported the letter to the police, noting that it was its first-ever bomb threat.

The statement said media coverage surged after it joined a collection of citizens’ groups seeking to add documents on the women for listing in UNESCO’s Memory of the World Register.

The citizens’ groups from countries including Japan, China and South Korea in June filed the application for 2,744 documents and other materials to be put into the UNESCO registry.