RAPID CITY, South Dakota (AP) – A man convicted in the 1975 slaying of an American Indian Movement activist will serve life in prison without parole, a judge decided Monday, closing a major chapter in an investigation that has spanned more than three decades.

John Graham, a 55-year-old Southern Tutchone Indian from Canada, was found guilty last month of felony murder for participating in a kidnapping that ended in Annie Mae Aquash's death. He's the second person convicted in Aquash's death, which garnered international attention and remains synonymous with the 1970s clashes between AIM activists and federal agents.

State law from the time of the incident, which prescribes a life sentence for felony murder without mentioning parole, requires a sentence of life without parole, state Attorney General Marty Jackley argued Monday. A South Dakota jury found Graham not guilty of premeditated murder.

Judge John Delaney agreed with Jackley but said arguments made by Graham's attorney, John Murphy, had “some merit.”

“The sentence is preordained at this point,” Delaney said, adding that he expected higher courts to make a final decision on what the law means. “The Supreme Court will decide this issue, not I.”

Authorities believe AIM leaders ordered her death because they thought she was helping the government, which officials have denied. No AIM leader has ever been charged in her slaying, and several people involved with AIM have denied their own involvement.

Federal and state prosecutors have confirmed that they continue to discuss the case, and the investigation remains open. On Monday, Jackley refused to say if more charges are forthcoming.

Aquash, a member of the Mi'kmaq tribe of Nova Scotia, was active in AIM and close to several of its leaders. By late 1975, however, she started to fear for her life due to circulating rumors that she was an informant, witnesses testified.

Graham did not testify at his trial, but spoke Monday before he was sentenced. Graham, wearing a striped jail uniform and chains around his waist and ankles, stood and turned to look at Aquash's daughters as he spoke. He accused witnesses of speaking in “half-truths.”

“The truth hasn't come out here,” Graham said. “Anna Mae was never kidnapped, never tied up in my presence, never murdered in my presence.”

Graham said he had taken Aquash from Denver to a safe house on Pine Ridge with her consent. He said he had not taken orders from anyone else. “I would not do that,” he said.

Denise Maloney Pictou, Aquash's elder daughter, spoke during the hearing about how the decades-long investigation strained her and her family, and accused AIM of bringing about Aquash's death. “They were no friends of my mother,” she said.

As she finished her statement, Maloney Pictou looked at Graham and held up a page-sized photo of her mother.

“This, John Graham, is what you stole from me,” she said.

Afterward, she was asked about Graham's statement. “I don't believe him,” she said.

Her sister, Debbie Maloney Pictou, described exhuming her mother's remains and reburying them in their native Nova Scotia.

“The images of her remains will stay with me forever,” Debbie Maloney Pictou said. “Because of you, I have held my mother's skull in my hands.”

Graham's brother, Harold Johnson, also spoke briefly, and Murphy submitted more than 20 letters on his client's behalf.

But Delaney said he would not change his mind about the sentence, though he acknowledged it would likely be examined by higher courts. Murphy has said he will appeal the conviction.

“None of that changes what happened 35 years ago,” Delaney said. “There's no way that Anna Mae comes back to life.”

During five days of testimony last month, witnesses said they saw Grahamand two other AIM activists take Aquash from a house in Denver and eventually to the Pine Ridge Indian Reservation. Arlo Looking Cloud, who was convicted in Aquash's slaying in 2004, testified that he watched Graham shoot Aquash with a .32-caliber pistol.

Murphy did not call any defense witnesses but questioned the credibility of Looking Cloud and others who said they saw the kidnapping.

AIM was started in the late 1960s to protest the U.S. government's treatment of American Indians and demand the government honor its treaties with Indian tribes. The movement grabbed headlines in the early 1970s with its takeover of the Bureau of Indian Affairs headquarters in Washington and its 71-day occupation of the South Dakota reservation town of Wounded Knee.