One of the principal leaders at the Whetstone Indian Agency, located along the Missouri River, where most of the Brulé and Oglala bands had gathered, Big Mouth gained increasing support for his stance among members of the tribe. He criticized what he described as Spotted Tail's reversal of Sioux policy, saying Spotted Tail had been entertained by American politicians and given a personal tour through the major cities of the east coast. Faced with increasing opposition to his leadership, Spotted Tail visited Big Mouth at his lodge, where, upon approaching the entrance, Big Mouth was seized by two warriors and held down while Spotted Tail shot and killed him.