According to her killer, Natalie Bollinger put a hit on herself on Craigslist.
Adams County Sheriff Michael McIntosh said Lopez was the sole suspect but that this didn't mean the investigation was over.
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 claims in the affidavit that he tried to talk Bollinger out of it, but that she wanted to end her life because of issues with her boyfriend.
Lopez told investigators Bollinger asked how much it would cost for him to kill her and he said he told her "from hundreds to thousands".
Police discovered the Broomfield teen's body off Riverdale Road on the property of McIntosh Dairy farm a day after her boyfriend reported her missing on December 28. He said Bollinger said she "wanted to get on her knees and be executed from behind because she did not want to see the gun", according to the affidavit.
Lopez said he and Bollinger knew each other through social media and had not been friends long.
The Denver station also reported that Lopez says they prayed together before the shooting.
The affidavit continues and states that Lopez arranged to pick up Bollinger from her Broomfield apartment on December 28.
Lopez told authorities that at no time did "they have or consume any type of drugs".
After more than 100 text messages between the two, they allegedly reached an agreement, and Lopez drove to Broomfield, picked the teen up and shot her in the back of the head execution style at her own request, Lopez claims in the affidavit.
They asked him Thursday as he arrived for work at Domino's Pizza if he was willing to answer some questions, the Denver Channel reported.
After the killing, investigators initially focused on Shawn Shwartz, a man who Bollinger had at one point obtained a restraining order against, according to the affidavit.
The affidavit stops short of providing evidence of Lopez's claims or describe how a dangerously high dose of heroin ended up in her system.
He then took off with Bollinger's purse and the gun, which he said was still in his trunk.
On Facebook, Bollinger's sister rejected Lopez's story, the Daily Beast reported.
The autopsy said Bollinger had a history of heroin and methamphetamine use.
McIntosh was adamant at Friday's news conference that investigators may not have uncovered the true motive.
Investigators have located a weapon, but McIntosh said it's too early in the investigation to tie it to Bollinger's murder. "If this is something that the victim was pursuing and asking for, it doesn't alleviate his legal responsibility, but it certainly mitigates it".
Lopez is being detained at the Adam's County Detention Center on first-degree murder charges.
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