What important event occurred ten years prior to the book's main narrative in the novel, The Only Good Indians?

One of the most important events in the novel takes place ten years prior to the main narrative when Lewis, Gabe, Cassidy, and Ricky go hunting elk on/near the reservation. When they see a large herd of elk move into a restricted area, they decide to pursue. The area is off-limits to hunting, and only Blackfeet elders are allowed to go there. The hunters kill several elk there. They are caught and are stripped of their hunting privileges.