A third man has been charged the 1975 shooting death of a Nova Scotia Mi'kmaq activist on an Indian reservation in South Dakota.

Richard Marshall of Rapid City, S.D., pleaded not guilty Tuesday to aiding and abetting the murder of Anna Mae Pictou-Aquash.

Pictou-Aquash was shot in the back of the head at the Pine Ridge Indian Reservation.

Marshall, who was a leader in the American Indian Movement (AIM), served 24 years in prison for another killing in 1975. He was paroled in 2000.

The movement occupied Wounded Knee on the Pine Ridge reservation during a standoff in 1973 that included a gunfire exchange with federal agents.

Fritz Ario Looking Cloud was convicted in 2004 and sentenced to life in prison for his part in the slaying.

Another man, Canadian John Graham, is scheduled to stand trial starting Oct. 6 in Rapid City.

Graham, who lived in Vancouver, was arrested for the murder in December 2003 and was subsequently extradited to the U.S. to face charges.

Graham and Pictou-Aquash were both AIM members at the time of the shooting.

Prosecution witnesses have previously testified that Pictou-Aquash, who was 30 at the time, was killed because AIM leaders thought she was a government spy.

With files from the Associated Press