Spell's alleged accomplice, Lester Van Waters Jr., 49, has a much more extensive criminal record. He pleaded guilty in a deal that lets him avoid a potential death sentence if he testifies against Spell.
Spell's attorneys say his intellectual disabilities date back to his elementary school days in Parachute, Colo. If he goes to trial, they say the death sentence sought by prosecutors should be barred by a U.S. Supreme Court ruling that forbids executions of the mentally disabled.
It's the first such argument in Montana since the high court's 2002 ruling in Atkins vs. Virginia, and prosecutors are fighting it. They are sending their own team to the state mental hospital to evaluate Spell, accused of randomly abducting and killing Arnold on Jan. 7, 2012.
Waters testified at his change of plea hearing that he and Spell had consumed massive amounts of cocaine as they drove from Colorado looking for work in the oil fields. He said they were looking for a woman to have sex with when they spotted Arnold along the side of a road in the small community of Sidney, a city of 5,000 near the confluence of the Yellowstone and Missouri Rivers.
Spell grabbed Arnold along an irrigation ditch just blocks from her house, overpowered and killed her, according to court documents filed by prosecutors. The remains of the 43-year-old mother of two were recovered from a shallow grave on March 21, 2012, near Williston, N.D., about 50 miles away.
In Sidney, pain over the death of a woman most refer to as simply "Sherry" is shared community-wide. Her tenure in the local school system stretched across two generations, and former students and their parents said they admired Arnold for her competence and compassion in the classroom.
"Our town is too wounded to be objective," said Tina Turner, a shop owner who searched for Arnold and pasted a missing person poster in the window of her Planet Hair & Spa.
"I've always believed every life is worth something, but when someone takes a life, it changes how you feel about things," she said.
Judge Richard Simonton moved the case from Sidney to Glendive, Mont., about 50 miles away, after the defense pointed to comments in the Sidney Herald that extolled "Old West Justice" and called for the accused to "hang from the nearest tree."
The trial, scheduled to begin Jan. 6, was canceled by Simonton on Wednesday after state officials said they needed until mid-January to evaluate Spell.
Arnold's sister, Rhonda Whited-Rupp, said her family believes justice was served when Spell and Van Waters were arrested.
"I want to drive over them, but we don't do that," said Whited-Rupp. "Death is not the ultimate goal. Just don't let them out, that's all I'm worried about, whether it's in prison or an institution or whatever."
Defense attorney Al Avignone said Spell reads at a first-grade level and has consistently scored below 75 on IQ tests, a cutoff intelligence level considered a central indicator of mental disability.
In 2012, authorities said Spell tried to lead FBI agents to Arnold's body but failed. Spell is illiterate, and his father has said he has less education than a kindergartner.
Filings by Richland County prosecutors showed that the state wanted its own specialists to evaluate Spell, setting up the potential for dueling experts when the matter comes back to Simonton.