Lessler said Curley's attorneys still have the option of making a conventional defense. An attorney for Curley did not respond to requests for comment.
Under Arizona law, an insanity defense would require Curley's lawyers show at the time of the crimes Curley did not understand what he was doing was wrong, said Zig Popko, a clinical professor of law at Arizona State University.
"It's strictly, 'He didn't know it was wrong to point the gun and shoot the victim,' " Popko said.
Defense attorneys would have the burden of proof, Popko said, and they would have to provide "clear and convincing evidence" that Curley did not know right and wrong. That's less of a standard than beyond a reasonable doubt but higher than a preponderance of evidence.
If a jury found Curley guilty but insane, the judge would then calculate how much prison time Curley would serve in a conventional conviction, Popko said. The judge would order Curley to serve that time under the jurisdiction of a state psychiatric board.
The board would incarcerate Curley in a mental health institution but could release him if Curley were able to demonstrate he no longer suffers from a disease or defect and is no longer a threat.
Insanity defenses, though, Popko said, are rarely successful.
Lessler said defense attorneys for Curley have not yet entered evidence of insanity into the public record. A trial will likely occur in 2012 in Flagstaff, Ariz.
On Aug. 26, police in Fredonia, Ariz., were trying to arrest Curley, who was suspected in burglaries at a school. Fredonia sits on the state line with Utah, and Harris responded to the call for assistance.
Harris and an officer were tracking Curley on foot through the redrock desert just miles inside the Arizona state line. Police have said Curley waited for the peace officers with a rifle and fired at them when they walked over a hill.
Curley was captured five days later a few miles from the shooting scene after a massive manhunt. Besides the murder charge, Curley is charged with two counts of aggravated assault, theft and burglary.