A Colorado man accused of killing a 19-year-old woman claims she hired him to kill her on Craigslist, Feb. 12, 2018. In this photo, the office of online site Craigslist is seen in San Francisco, California, March 10, 2006. Getty Images

A Colorado man accused of shooting and killing a teenage girl told police he shot her after he replied to her Craigslist ad putting a hit out on herself, asking to be murdered, investigators said in court documents obtained Monday.

Joseph Michael Lopez, 22, was arrested last Thursday in connection to the death of Natalie Bollinger. Lopez apparently tried to talk her out of suicide, he told Adams County deputies, according to an arrest affidavit obtained by NBC News on Monday.

Lopez told investigators that sometime after Christmas, he reportedly came upon an ad on Craigslist titled "I want to put a hit on myself," while looking at the "Woman seeking Men" category, according to the affidavit. Lopez stated the ad was posted by Bollinger.

The Adams County Sheriff's Office did not confirm whether such an ad was actually placed, a spokesman told NBC News on Monday.

According to the arrest affidavit, Lopez claimed he responded to the ad as he had been a victim of depression and had suicidal thoughts when he was in high school and thus thought he could help Bollinger.

Friends and relatives told investigators Bollinger had a history of suicidal thoughts. However, they said they had found no suicide note or any other communication indicating she wanted to take her life. The affidavit even stated she had applied for a job Dec. 28, the day before she was killed.

Lopez claimed the teen could not be persuaded, telling investigators she said she wanted to be executed from behind so she would not have to see the gun and that she "wanted it to happen quickly," according to the affidavit.

He also told investigators after he failed to change the teenager's mind, they met and went for a drive and stopped by the side of a road, where they first reportedly said a prayer together and then he claimed she "wanted to be on her knees and executed from behind." Lopez added he used a gun the teen had given him, which she reportedly said he could keep as payment.

An autopsy result conducted by the Adams County coroner claimed Bollinger died from a gunshot wound to the head. The autopsy report also stated the victim had a history of heroin and methamphetamine use, and mentioned at the time of her death, she had a "potentially lethal level of heroin in the blood."

The body of Bollinger, who lived in Broomfield, was found along a dirt road by a passerby Dec. 29 in an unincorporated area of Adams County, east of Boulder, Colorado.

Joseph Michael Lopez claimed the teen could not be persuaded, and added she said she wanted to be executed from behind so she would not have to see the gun. Above is a representational image of a crime scene tape and police cars. Getty Images

Lopez is being held without bail Monday in Adams County jail on suspicion of first-degree murder. His court hearing is expected to be held Wednesday.

He was said to have left from the scene of the crime with Bollinger’s purse and the gun. Adams County Sheriff Michael McIntosh said Friday, when the arrest was announced, they were able to track down Lopez through dozens of text messages on the victim’s cellphone.

"A 19-year-old's phone, there's a lot of data there," McIntosh said. He added no one else is believed to have been involved in the incident.