Serial shooting suspect John Delling might be mentally fit to stand trial for murder this spring, but his attorneys say Delling wasn't mentally healthy enough to know what he was doing when he shot and killed Brad Morse in 2007.

That sounds a lot like an insanity defense - a plausible way to explain to a jury how someone who has been diagnosed with severe paranoid schizophrenia might not have been in control of himself when he traveled across 6,500 miles of the Western United States in 2007, shot three acquaintances and killed two of them.

Delling and his attorneys have a major problem offering this strategy, however. Though mental stability can lighten a person's sentence, Idaho is one of three states that do not allow an insanity defense to try to beat a conviction.

That didn't stop Delling's attorney, public defender Gus Cahill, from filing a motion last week telling 4th District Judge Deborah Bail that he planned to put on psychological evidence during the upcoming trial that "Delling was incapable of forming mens rea" at the time of Morse's murder.

Mens rea is a legal term for malice aforethought - the thoughts and intentions behind a wrongful act, including knowledge that the act is illegal, according to a legal definition provided by Princeton University. In Latin, mens rea is literally "guilty mind."

So how can Delling's attorneys pursue what appears to be a defense not allowed in Idaho?

Very carefully, says Boise-based defense attorney David Leroy, who was Idaho's attorney general when the state did away with the insanity defense in 1982.

"It's a very fine line," Leroy said. "I think they must (argue), in a narrow and focused way, that his mind was incapable of forming a specific element of the crime charged, such as premeditation or malice aforethought."

The Idaho Legislature banned the use of insanity as a defense amid the national outcry over the acquittal of would-be presidential assassin John Hinckley Jr., who shot President Ronald Reagan in 1981.

Montana and Utah also ban the insanity defense, according to the American Bar Association.

"Idaho eliminated the insanity defense in the old English common law sense. That is the argument that the defendant was out of touch with reality and didn't understand the consequences of their actions," Leroy said. "We determined in 1982 that a better test (for insanity) would be to ask the jury to examine the specific mental state of the defendant at the time of the crime for the presence or absence of a specific element.

"Since the prosecutor must prove all elements of the crime to convict, the absence of such proof as to a mental element still constitutes a defense."

Boise defense attorney Tom McCabe said providing psychological evidence could have a two-pronged benefit for a defendant like Delling:

The jury might agree on a lesser charge, like voluntary or involuntary manslaughter, which doesn't have the malice aforethought requirement that first- or second-degree murder does.

And the prosecutor could consider the "mens rea" issues problematic enough to want to avoid a trial altogether and pursue a plea agreement. By charging Delling with first-degree murder, Ada County prosecutors are saying they can prove to a jury that Delling was mentally capable of premeditating the murder of Morse.

"The insanity defense allowed you to get an acquittal - Idaho decided we can't have that after Hinckley," McCabe said. "But they can't forbid you from using psychological evidence."

Evidence of a mental illness is often used as a mitigating factor in sentencing for murder cases - which can keep a convicted killer off death row. Prosecutors have already decided not to pursue the death penalty for Delling.

Both Idaho and federal law require that defendants must be mentally competent to stand trial in the first place.

Delling's own competency wasn't established until Feb. 19, when Judge Bail issued an order saying Delling was finally mentally fit to stand trial.

Delling had been in the custody of the Idaho Department of Health and Welfare since February 2008, and it took three psychiatric evaluations before Bail decided he was fit to assist in his own defense and to make informed decisions about his medical treatment.

Patrick Orr: 373-6619