SIOUX FALLS, S.D. (AP) – A Denver man convicted of first-degree murder for his role in the 1975 shooting death of an American Indian Movement activist has had his federal prison sentence reduced from life to 20 years, federal court documents show.

In February 2004, a federal jury in Rapid City, S.D., convicted Arlo Looking Cloud, 58, in the slaying of fellow AIM activist Anna Mae Pictou Aquash, a 30-year-old member of the Mi'kmaq tribe of Nova Scotia. He was sentenced to a mandatory life prison term.

But Looking Cloud in December testified for state prosecutors against co-conspirator John Graham, whom jurors convicted of felony murder.

A series of court filings in Looking Cloud's federal case since March have been sealed, but documents show that U.S. District Judge Lawrence Piersol signed an order in August reducing Looking Cloud's sentence.

Mark Salter, a spokesman for the U.S. Attorney's Office in South Dakota, said he couldn't offer any additional information other than what was in the signed amended judgment.

A call to Looking Cloud's attorney was not immediately returned Monday.

Aquash's frozen body was found in February 1976 on the Pine Ridge Indian Reservation. The Canadian woman had been shot in the head.

Federal agents investigated for years but didn't bring an indictment until March 2003, when Denver police arrested Looking Cloud.

Graham, of the Southern Tutchone tribe in Canada's Yukon territory, was arrested in December 2003 in Vancouver, British Columbia, on federal charges in Aquash's killing. But two courts ruled that the U.S. government lacked jurisdiction to try Graham because he is not American Indian, and the case was eventually moved to state court.

In Graham's December trial, Looking Cloud testified that he stood nearby as Graham shot Aquash on the reservation and left her to die.

Graham's attorney, John Murphy, suggested that Looking Cloud had embellished his story to get his life sentence reduced. Looking Cloud agreed that he had left out details before, but repeatedly said he was trying to tell the truth in the testimony.

Aquash's elder daughter, Denise Maloney Pictou, said Monday that she had no comment on the sentence reduction.

AIM was founded in the late 1960s to protest the U.S. government's treatment of Indians and demand the government honor its treaties with Indian tribes. The group grabbed headlines in 1973 when it took over the village of Wounded Knee, leading to a 71-day standoff with federal agents that included the exchange of gunfire.

Prosecutors believe Graham, Looking Cloud and a third AIM activist kidnapped and killed Aquash because AIM leaders thought she was a government spy.