AIM became involved in the political faction wanting to oust Richard Wilson, the elected chairman of the Oglala Sioux Tribe. Opponents believed that he was acting autocratically, including recruiting a private police force. A failure of an impeachment proceeding against him led to a large protest. Banks and other AIM activists led an armed takeover and occupation of Wounded Knee. After a siege of 71 days by federal armed law enforcement, which received national attention, the occupation was ended. A U.S. marshal was shot and paralyzed in March. A Cherokee and an Oglala Lakota were fatally shot in April 1973. Civil rights activist Ray Robinson, who had joined the protesters, disappeared during the occupation and is believed to have been murdered.