The man charged in the shooting death of 19-year-old Natalie Bollinger allegedly told Colorado investigators she hired him to kill her.

Joseph Lopez, 22, was arrested last week and charged with first-degree murder. According to an arrest warranted obtained by PEOPLE, Lopez told police that he responded to an ad that Bollinger allegedly posted on Craigslist, entitled “I want to put a hit on myself.”

Bollinger was reported missing days after receiving a protection order against a man she alleged was harassing her. She was found dead in a wooded area late last year. An autopsy also revealed she had a “potentially lethal level of heroin” in her system at the time of her death.

“Just because we have a suspect in custody doesn’t mean the investigation is over,”Adams County Sheriff Michael McIntosh told reporters on Friday, adding that Lopez is the sole suspect Bollinger’s slaying.

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According to the arrest affidavit, detectives uncovered more than 100 text messages between Lopez and Bollinger.

The affidavit alleges that Lopez told police that “he was pretty sure that he knew what we were there for” and that “he was sure it had to do with the girl he talked to on Craigslist.”

Lopez said that he spotted the Craigslist ad in late December. He allegedly said he contacted the teen, pretending to be an experienced hired killer. Lopez allegedly said he agreed to meet with Bollinger and kill her after the two spoke via text.

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He allegedly arranged to pick her up at her apartment on December 28, 2017. The affidavit states that they discussed payment. Bollinger allegedly wanted to be killed “on her knees… executed from behind,” reads the affidavit. Lopez alleges Bollinger provided him with the gun he used to end her life. He claims that he tried to change her mind, but she insisted.

Bollinger was reported missing by her boyfriend; he also claimed his gun went missing.

At first, Lopez allegedly said he had a change of heart, and that he dropped Bollinger off at her apartment where she shot herself. Later, he allegedly confessed, telling investigators Bollinger “knelt down on the ground and that he knelt down along her left side and slightly in front of her.”