For the first time since marijuana was legalized in Colorado, defense attorneys are poised to argue that cannabis intoxication played a key role in a homicide.

A recent set of filings by Richard Kirk’s attorneys — and his change of plea to not guilty by reason of insanity — provide a possible preview of the connection attorneys may try to make between Kirk’s mental state the night he killed his wife and the nibbles of a marijuana candy he ate hours before he pulled the trigger.

While legal experts say the role of intoxication in insanity cases in Colorado is nothing new, the involvement of the newly legal drug in one of the city’s most prominent cases will continue to draw attention locally and nationally.

“You just don’t see the correlation between marijuana and homicide that you do with the strong correlation between alcohol and homicide, the strong correlation between amphetamines, cocaine and homicide,” said Denver attorney and legal analyst Christopher Decker.

Kirk, now 49, is charged with one count of first-degree murder in the death of his wife, 44-year-old Kristine Kirk in their Observatory Park home on April 14, 2014.

Kristine Kirk called 911 and pleaded for help as she told the operator her husband was ranting about the end of the world, asking her to shoot him and “totally hallucinating.” More than 12 minutes after she called police, what sounded like a gunshot was heard, followed by silence.

DOCUMENTS: Richard Kirk’s psychological evaluations, medical opinions and more

The couple’s three young sons were home at the time.

The partially eaten marijuana candy Richard Kirk nibbled on, and an untouched joint, were found inside the home.

Five weeks before Kirk’s trial was set to start, he changed his plea from not guilty to not guilty by reason of insanity. But as the crux of the case shifts, defense attorneys will have to work to keep the focus on Kirk’s mental state at the time of the shooting and not just the marijuana, Decker said.

“It makes it that much more challenging to raise those issues of sanity with marijuana given the political nature of the marijuana laws and the public opinion of that,” Decker said. “It’s going to boil down to the expert opinions.”

By the time Kirk changed his plea Sept. 18, a forensic psychiatrist, psychologist, toxicologist and an expert on the effects of marijuana had examined the case. Kirk will now be sent to the Colorado Mental Health Institute in Pueblo, where a court-appointed forensic psychiatrist will complete an evaluation.

For a jury to decide Kirk was insane at the time of the shooting, the panel must find that he had an underlying mental condition that prevented him from understanding the difference between right and wrong.

“What we’re seeing is that it is more complicated than simply this defendant consumed marijuana and killed his wife as a result,” said Sam Kamin, Vicente Sederberg professor of marijuana law and policy at the University of Denver. “The assertion of the insanity defense is an indication that the defense team is asserting there was a deeper problem going on.”

Any kind of intoxication, in itself, is not an accepted defense, but it can call into question the mind-set of the defendant, Kamin said.

Defense attorneys repeatedly have argued that on the night of the shooting, Kirk did not have the mental capacity to plan to kill or to kill his wife intentionally.

Prosecutors have rejected any arguments that Kirk was not acting intentionally. They claim the financial and emotional strain of his marriage contributed to a conscious decision to kill his wife.

Hours before the shooting, Kirk purchased “Karma Kandy Orange Ginger,” a candy form of edible marijuana. The candy — which was roughly the size of a Tootsie Roll — contained 10 servings of THC, with each serving containing about 10 milligrams.

During a preliminary hearing, attorneys said blood tests revealed Kirk had a low level of THC — marijuana’s psychoactive ingredient — in his system. No alcohol or other drugs were detected.

A defense motion included a toxicology report from one of Kirk’s blood tests the night of the shooting. It showed he had 2.3 nanograms of THC per milliliter of blood.

The state’s legal limit for stoned driving is 5 nanograms of THC per milliliter.

Ahmed White, who teaches criminal law at the University of Colorado, said intoxication cases are nothing new. He is unaware of anything that would prevent the same standards used in cases with alcohol or other drugs from being applied in the Kirk case.

On Sept. 1, Kirk’s attorneys filed a motion disclosing they may argue Kirk’s actions were the result of intoxication or involuntary intoxication.

They also submitted three reports from experts they hired to evaluate the case. The reports — when considered together — suggest the THC in Kirk’s system may have contributed to or triggered a psychotic break.

The first report was completed by Ryan Vandrey, who studies the effects of marijuana at Johns Hopkins University’s School of Medicine. Vandrey said his research shows people who eat pot may experience strong effects but may show relatively low levels of THC in their blood.

Vandrey also wrote he has never seen someone react the same way Kirk did after eating marijuana, but he is aware of cases in which people experience psychotic behavior after eating pot.

The next report went one step farther.

Dr. Andrew Monte a medical toxicologist and emergency physician at the University of Colorado Hospital concluded that Kirk’s THC intoxication led to “delirium and psychotic-like symptoms.”

But he did not go so far as to say that Kirk has an underlying mental condition that was triggered by the THC.

The third report was written by Katherine Bellon, a clinical psychologist and the only person to speak with Kirk before writing a report. Bellon found that as a result of his mental condition, Kirk may be vulnerable to “serious distortions in thinking.”

The report did not draw a conclusive line between Kirk’s consumption of pot and his actions in 2014.

Jordan Steffen: 303-954-1794, jsteffen@denverpost.com or @jsteffendp