RAPID CITY, S.D. (AP) – A former suspect in the slaying of an American Indian Movement activist must testify in the upcoming trial of the alleged shooter, a judge ruled Tuesday.

Richard Marshall was found not guilty earlier this year of providing the gun used to kill Annie Mae Aquash in 1975. Now, prosecutors want Marshall to testify at the trial of John Graham, who has pleaded not guilty to shooting Aquash and leaving her to die on South Dakota's Pine Ridge reservation.

Marshall's testimony could offer new insight into how and why Aquash was killed. The slaying became synonymous with the American Indian Movement and its clashes with federal agents during the 1970s. Prosecutors have offered Marshall immunity, but his attorney, Dana Hanna, argued Monday that state immunity wouldn't protect Marshall from prosecution in tribal courts, which are separate.

In a Monday hearing, Delaney suggested he shared some of Hanna's concerns. But on Tuesday, Circuit Judge John Delaney said he had found out Marshall was protected in Pine Ridge tribal court by its statute of limitations, which specifies the amount of time prosecutors can charge someone after a crime occurs.

Delaney said Pine Ridge's statute of limitations was one year – while Aquash's murder occurred about 35 years ago – and courts in other states where Aquash was kidnapped and taken would have to abide by South Dakota's immunity pledge.

"It would be unconstitutional for them, after immunity is granted,'' to prosecute Marshall, Delaney said.

Marshall said Monday he intended to use his Fifth Amendment right not to incriminate himself by testifying. Delaney said state immunity made that unnecessary. "I believe the immunity given to Mr. Marshall is sufficient to protect him,'' he said.

Hanna then said prosecutors only wanted Marshall to testify so they could charge him with perjury, a charge that's not blocked by the state's offer of immunity. Delaney said he would hold a hearing Wednesday morning to decide that issue.

Jury selection in Graham's trial continued Tuesday, with opening statements expected Wednesday. The trial will likely last about two and a half weeks.

Graham faces one count of felony murder in relation to kidnapping, one count of felony murder in relation to rape and one count of premeditated murder in Aquash's slaying. The charges carry a maximum penalty of life in prison.

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. It first gained national attention in 1972 when it took over the Bureau of Indian Affairs headquarters in Washington.

The next year, the group occupied Wounded Knee – the site of the 1890 standoff between Lakota Indians and U.S. Army forces in which hundreds of Lakota were killed. AIM controlled Wounded Knee for 71 days from late February to May of 1973 before surrendering to federal agents.

Aquash, a native of the Mi'kmaq tribe of Nova Scotia, took part in Wounded Knee and remained active within the group afterward. But rumors began to circulate that Aquash might be a government informant, particularly as she became more involved within AIM, witnesses have said.

Aquash was 30 years old when she was killed by a gunshot wound to the head near Wanblee.