However, at that time the cause was unclear; and in Choya Shinbun, the newspaper that first reported the incident on August 12, reported ambiguously that Ashio copper mine may have caused it.
ただし、当時は原因が分かっておらず、これを8月12日に最初に報じた朝野新聞も、足尾銅山が原因かもしれないというような、あいまいな書き方をしている。