BILLINGS — Attorneys for a Colorado man charged with killing an eastern Montana teacher filed court papers Friday saying his mental disabilities render him unfit to stand trial in a case that could carry the death penalty.

Defense attorneys asked state district Judge Richard Simonton to commit 24-year-old Michael Keith Spell to the Montana State Hospital or another state facility for up to 60 days to undergo a mental examination.

The Parachute, Colo., man faces a felony murder charge in the January 2012 death of Sherry Arnold, a popular Sidney High School math teacher.

Court records obtained by The Associated Press show that Spell was declared incompetent by a Colorado judge in 2010 in a Garfield County, Colo., drug case. A subsequent order for Spell to undergo treatment was still pending at the time of his arrest in the Arnold murder.

Prosecutors in Montana have said they intend to seek the death penalty for Spell if he’s convicted.

But Spell’s attorneys have mounted an aggressive defense to spare their client execution. Those efforts are centered on a contention that under state law, Spell’s mental problems require the courts to treat him differently than a typical defendant.

His attorneys also say Spell has a prior diagnosis from psychologists of being intellectually disabled.

“At the time of Sherry Arnold’s death, Mr. Spell was intellectually disabled,” attorneys Al Avignone and Lisa Banick wrote in Friday’s request for him to be committed. “His intellectual disability rendered him unable to appreciate the alleged criminality of his behavior, or to conform his behavior to the requirements of the law. Mr. Spell’s intellectual disability also renders him unfit to proceed.”

Co-defendant Lester Van Waters Jr. pleaded guilty in August.

A deal with prosecutors calls for him to be spared the death penalty and receive 100 years in prison in exchange for testifying against Spell.

Arnold’s sister, Rhonda Whited-Rupp, said Friday that she believes justice will prevail.

“My opinion is that it’s a blessing they both were caught,” she said. “God will take care of the end result.”

The Richland County Attorney’s Office has opposed a defense motion to declare Spell ineligible for the death penalty, under a 2002 Supreme Court ruling that forbids states from executing the mentally or intellectually disabled under the prohibition against cruel and unusual punishment in the Eighth Amendment.

Documents filed by prosecutors opposing that motion have been sealed by the court.

Spell and Van Waters came to Montana looking for work in the Bakken oil patch along the North Dakota border.

Waters said during an August change of plea hearing that the pair consumed large amounts of cocaine during their drive from Colorado. They were in Sidney looking for a prostitute to have sex with when they saw Arnold who was out on her morning run, Waters said.

Authorities say Spell killed the 43-year-old teacher by either choking her or holding her face in a puddle.

Spell and Van Waters later buried her body near Williston, N.D. It was not found for more than two months, when Waters led FBI agents to the site and alleged that Spell was the killer.

The case spurred federal prosecutors to convene a retreat last year for law enforcement to craft a strategy to deal with rising crime in the Bakken region.

Spell’s trial is slated to begin Jan. 6 in Glendive, where it was relocated from Sidney after Avignon and Banick claimed their client would not get a fair trial in Arnold’s hometown.