( 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 did n'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 do n'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 ca n'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 .
NEW : Authorities charge Richard Kirk with first-degree murder in his wife 's death . Denver police say he shot and killed his wife while she was on call to 911 . Kristine Kirk told 911 operator husband was hallucinating after he had `` taken some marijuana '' Colorado is first state in the nation to legalize the sale of recreational marijuana .