RAPID CITY, S.D. (AP) – The acquittal this week in the slaying of an American Indian Movement activist highlights the challenges of trying a nearly 35-year-old case as the alleged triggerman is scheduled to go on trial this summer.

Jurors found Richard Marshall not guilty Thursday of first-degree murder. He was accused of providing the weapon that killed Annie Mae Aquash in 1975.

A key federal agent during the militant group's protests in South Dakota at the time of the slaying says the acquittal gives him pause.

“Well, it's not very positive, that's for sure, because you never like to lose them,” said Norman Zigrossi, director of Rapid City's FBI office in the 1970s. “But it's hard to say. A lot of it depends on the jury, a lot of it depends on how the facts are presented. Not witnessing the trial, it's really hard for me to pass judgment on what happened.”

Aquash came from Canada in the 1970s to join the American Indian Movement and pursue its complaints against poverty and racism. Witnesses say Aquash was executed in 1975 because some AIM leaders thought she was a secret agent. Federal authorities deny that she was a spy.

The case was revived in 2003 when Arlo Looking Cloud and John Graham were indicted by a federal jury. Looking Cloud was found guilty of murder in 2004 and sentenced to life in prison. Graham's case was eventually moved to state court, where he stands accused of shooting Aquash in the back of the head.

Graham's lawyer, John Murphy, declined to be interviewed Friday.

Authorities believe Marshall gave the gun to Looking Cloud, Graham and another AIM member on the night Aquash was killed. The jury rejected that idea Thursday in less than two hours, which included lunch.

“I can't believe it was such a short deliberation,” Zigrossi said. “That is unusual.”

“I would have expected that (a guilty verdict) was a slam dunk because the facts are the same as Looking Cloud's trial,” he said.

Shortly after Marshall walked out of a county jail Thursday following his acquittal, a spokesman for the U.S. attorney's office in South Dakota acknowledged the difficulty of trying a nearly 35-year-old murder case. South Dakota Attorney General Marty Jackley said Murphy's case is different from Marshall's, and said the trial should go on as scheduled in July.

Dana Hanna, Marshall's attorney, argued there was a contradiction in facts. Looking Cloud, the government's key witness, didn't tell authorities about Marshall's alleged role in the killing until 2008. Looking Cloud said he kept quiet because he was afraid of Marshall. Hanna said Looking Cloud changed his story to get out of prison.

Hanna didn't return phone calls from The Associated Press. A daughter of Aquash who attended most of the trial, Denise Maloney Pictou, also didn't return phone messages seeking comment.

Owen Marshall, whose father is a cousin of Richard Marshall, said he believes the verdict was just, but says he feels for Aquash's family.

“I believe the person who shot her will be brought to justice,” said Marshall, who was born four years after Aquash was killed.