(CNN) -- What role, if any, did legally purchased and consumed marijuana play in a husband's alleged murder of his wife this week in Denver?
That's what police are investigating in the shooting death of Kristine Kirk, who called 911 Monday and, according to a police affidavit, said her husband, Richard Kirk, was hallucinating after he had "taken some marijuana."
"This is under investigation," Denver police spokeswoman Raquel Lopez said about the possibility of marijuana use.
Kristine Kirk, 44, told a 911 operator that her husband also may have been on some prescription medication for back pain, according to a police affidavit.
The affidavit, written by a police detective, recounts details of the 13-minute 911 call that ended with what the detective described as "the sound of a gunshot."
According to the affidavit:
Kristine Kirk told the 911 operator that her husband was speaking ominously about "the end of the world" and said he wanted her to shoot him.
She said she feared for herself and her three children, who were in the house.
"At one point during the call (she) sounds panicked and tells the 911 operator that Richard was taking the firearm out of the safe," the affidavit says.
Seconds later the 911 operator heard screams and there was a gunshot sound. The call went silent.
Police arrived at the home to find Kristine Kirk dead, a gunshot wound to her head.
As an officer put Richard Kirk in the backseat of a patrol car, he admitted "without questioning" that he killed his wife, a police document says.
On Friday, Denver District Attorney Mitch Morrissey charged Richard Kirk with first-degree murder in his wife's death.
It could not be determined Friday whether Kirk had retained an attorney on his own. The Denver public defender's office didn't immediately return a call.
Investigators took a blood sample to determine whether Kirk was under the influence of any substances at the time of the shooting. But even if those blood tests reveal traces of tetrahydrocannabinol -- or THC, the psychoactive ingredient in marijuana -- some are saying it's unfair and irresponsible to blame the killing on his alleged use of marijuana.
"I think there are a lot of facts that we don't know about this story" said Rachel K. Gillette, executive director of Colorado NORML -- the National Organization for the Reform of Marijuana Laws. "Bottom line: any drug, whether it's caffeine, nicotine, alcohol, cocaine, prescription drugs -- they all come with certain levels of health risks. That's one of the reasons we're right to be regulating (marijuana)"
During a search of the Kirk's home, police found a receipt in the basement for purchases made on the day of the killing at a marijuana dispensary called Nutritional Elements, according to a warrant.
Kirk spent $32.70 on two items: Karma Kandy Orange Ginger -- a type of candy-form marijuana -- and Pre 98 Kush Pre-Roll -- a pre-rolled joint.
Colorado became the first state in the nation to legalize the sale of recreational marijuana, allowing pot stores to open for business on January 1.
A Nutritional Elements employee hung up immediately when a CNN reporter identified himself Friday.
"As tragic as this is" said Gillette, "we can't look at this one isolated incident and say that (legalizing marijuana) was the wrong path to go down. The best way to keep marijuana safe is for regulatory oversight and testing."
Richard Kirk is being held without bond in the Denver Detention Center.
CNN's Carma Hassan contributed to this report.
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Posted by: Nick Viviani