Woodland Park Police Department(DENVER) — Missing Colorado mom Kelsey Berreth’s parents —- who are suing son-in-law Patrick Frazee for the wrongful death of their daughter —- allege in new court documents that Frazee murdered his fiancée because she refused to give him full custody of their one-year-old daughter Kaylee.“Upon information and belief, Frazee had motive to kill Kelsey in that he wanted full custody of [Kaylee Berreth] and/or Kelsey to leave [Kaylee Berreth] with him and Kelsey would not agree,” the parents’ attorney, Angela Jones, wrote in an amended civil complaint filed Friday in federal court on behalf of plaintiffs Cheryl and Darrell Berreth.The lawsuit claims Frazee told Cheryl Berreth a series of lies “knowing that Kelsey was dead because he had killed her, or caused her to be killed, on November 22, 2018.”On December 2, the plaintiffs claim, Cheryl Berreth called Frazee and asked him if everything was “okay”.Frazee allegedly told Cheryl that he and Kelsey had broken up on Thanksgiving Day and agreed to split custody of their daughter. That same day, Kelsey left Kaylee with Frazee while she “figured out what she was going to do”,” according to the phone call documented in the amended complaint.Kelsey also allegedly asked Frazee “for her things back, so he gave Kelsey her keys and her gun,” according to the complaint.According to Cheryl, Frazee told her that he and Kelsey had plans for November 25, three days after she was last seen shopping at a local supermarket in Woodland Park, Colorado.“When he later sent a text and she didn’t respond, he figured she had put her phone on do-not-disturb so that she could study, which is something he said that she often did when she was studying or [Kaylee] was napping,” the court documents detail.Frazee allegedly also told Cheryl that despite the relationship starting off well, it eventually turned sour. He had “had enough and wasn’t going to deal with things anymore,” because Kelsey was always criticizing him or “putting him down” in front of the baby.In explaining one possible reason for her disappearance, Frazee allegedly suggested to Cheryl that Kelsey may have disappeared with a friend or co-worker because “Kelsey didn’t always return home directly after she got off of work and that she had gone out to dinner with some co-workers,” according to the court documents.Frazee allegedly also told Cheryl that despite the relationship starting off well, it eventually turned sour. He had “had enough and wasn’t going to deal with things anymore,” because Kelsey was always criticizing him or “putting him down” in front of the baby.In explaining one possible reason for her disappearance, Frazee allegedly suggested to Cheryl that Kelsey may have disappeared with a friend or co-worker because “Kelsey didn’t always return home directly after she got off of work and that she had gone out to dinner with some co-workers,” according to the court documents.On December 3, “Frazee called Cheryl-Lee Berreth and told her he hadn’t been able to access Kelsey’s phone records online. He stated that she had set up the online access when she was working in Grand Junction and he didn’t know the answers to the access questions like ‘where did you meet your spouse?’”Frazee also stated that he thought that Kelsey may have opened her own phone plan and changed her phone number. He told Cheryl-Lee Berreth: ‘I love your daughter.’ He said that Woodland Park is a safe place and the Berreths didn’t need to worry about foul play,” according to the documents.The documents also allege that Frazee encouraged false reports about Kelsey, including that the couple were not engaged, that Kelsey was not Kaylee’s primary caregiver, that Kelsey had gone to rehab, that she had run off before, that she had abandoned the baby in Frazee’s care and that she “had ‘issues’ that would warrant Frazee “getting full custody.”Kelsey Berreth was last seen in public Thanksgiving Day. Police arrested Frazee December 21. Even though her body has not been found, Frazee has been charged with her murder. He has not entered a plea.On February 8, Idaho nurse Krystal Lee Kenney pleaded guilty to helping Frazee dispose of Berreth’s cell phone. She is cooperating with the investigation and is required to testify against Frazee as part of her plea deal with prosecutors.Cheryl and Darryl’s newly amended complaint even go as far as to venture a guess as to how Frazee may have allegedly murdered their daughter.“Frazee had an opportunity to kill Kelsey or have Kelsey killed in that he had her keys and, because he had her gun, she was vulnerable to an attack,” the documents stated.Calls to the attorneys representing Patrick Frazee in the civil lawsuit have not been returned.Last month, a judge granted temporary custody of Kaylee to Berreth’s parents, Cheryl and Darrell Berreth. The couple also has filed a wrongful death lawsuit against Frazee.Frazee is due back in court Feb. 19.Copyright © 2019, ABC Radio. All rights reserved.