Sioux Falls, S.D. ? In a case that has haunted South Dakota for nearly 30 years, police have arrested a man in the slaying of an American Indian Movement activist whose frozen body was found on the Pine Ridge reservation in 1976.

Authorities said Arlo Looking Cloud, 49, was arrested in Denver last week. He pleaded innocent Monday to a charge of first-degree murder in the death of Anna Mae Pictou-Aquash, U.S. Atty. James McMahon said Wednesday in Sioux Falls.

Pictou-Aquash, 30, disappeared in late 1975 from a Denver home where she had been staying. Her body, with a gunshot wound to the head, was found in February 1976 on the sprawling reservation 90 miles east of Rapid City.

AIM leaders often cite the case and other unsolved slayings to suggest federal authorities don’t aggressively pursue murders on reservations.

Looking Cloud worked as a security guard at AIM events during the 1970s, said Paul DeMain, editor of the bimonthly newspaper News From Indian Country in Wisconsin.

AIM was beset by internal disputes at the time, DeMain said.

Pictou-Aquash, a member of Canada’s Mi’kmaq Tribe, was among the Indian militants who occupied the village of Wounded Knee in a 71-day standoff with federal authorities in 1973.

Some speculated she was killed by AIM members because she knew some of them were government spies, while others said Pictou-Aquash was killed because she herself was an informant.

Federal authorities have repeatedly denied any involvement.

A hearing was planned for today to determine whether Looking Cloud should be brought to South Dakota to face charges. If convicted, he would face a sentence of life in prison.