BILLINGS — A Colorado man charged in the murder of an eastern Montana teacher was diagnosed with mental disability following a recent evaluation at Montana State Hospital, according to court documents filed Thursday by defense attorneys.

Attorneys for 24-year-old Michael Keith Spell have argued that their client’s disability, including a limited ability to read or write, makes him unfit to be tried in the January 2012 killing of 43-year-old Sherry Arnold of Sidney. Arnold was taken while jogging near her home. Her body was found buried in North Dakota.

Spell is charged with deliberate homicide and attempted kidnapping. He could face the death penalty if convicted.

In Thursday’s court filing, defense attorneys Al Avignone and Lisa Banick said a Jan. 29 report from two doctors at the state hospital states that Spell was diagnosed as disabled after a two-month, court-ordered evaluation.

The filing did not detail whether the state’s doctors found him incompetent.

But Spell’s attorneys wrote that the doctors’ opinions regarding Spell’s fitness to proceed were “unfounded and not reliable” — suggesting they run counter to defense arguments that he should avoid trial.

That’s sure to be a point of contention during an upcoming competency hearing in Sidney. Richland County Attorney Mike Weber, who is prosecuting the case, did not immediately return telephone messages seeking comment.

The defense also said a juvenile case against their client was dismissed in Colorado in 2007 at the request of the prosecutor due to Spell’s incompetency.

The Associated Press has previously reported another incompetency finding for Spell in a 2010 Colorado drug case.

At least two doctors, psychiatrist Virginia Hill and psychologist Timothy Casey, examined Spell at the state hospital in Warm Springs, which is overseen by the Montana Department of Public Health and Human Services. Agency spokesman Jon Ebelt said he could not discuss Spell’s case because of patient confidentiality rules.

A co-defendant, Lester Van Waters, Jr., pleaded guilty last year in a deal that would allow him to avoid the death penalty in exchange for testimony against Spell.