The parents of missing Colorado mother Kelsey Berreth claim that her fiancé, Patrick Frazee, killed their daughter because of a dispute over the full custody of their one-year-old child. Over the weekend, Berreth's parents updated a wrongful death lawsuit against Frazee, saying the custody dispute was likely a "motive to kill Kelsey."

“Upon information and belief, Frazee had motive to kill Kelsey in that he wanted full custody of Kaylee and/or Kelsey to leave Kaylee with him and Kelsey would not agree,” the parents' attorney, Angela Jones, wrote in the amended civil complaint filed on behalf of Kelsey’s parents, Cheryl and Darrell Berreth in January.

The parents said they were concerned about their daughter's well-being after she didn't respond to calls or texts for a week before her disappearance. So Cheryl called Frazee on Dec. 2 to ask about her daughter.

"Frazee responded with 'here's the story...'" and proceeded to make a series of "false statements, misrepresentations, and/or calculated omissions," the court filing states, adding that Frazee told Cheryl he and Kelsey broke up on Thanksgiving, and that she "agreed to share with Frazee 50/50 custody of their daughter." 

When asked in detail about what had happened, "Frazee suggested that Kelsey may have flown somewhere with a friend or co-worker," the amended complaint states.

Kelsey was last seen shopping on Thanksgiving Day and three days later her cell phone pinged near Gooding, Idaho, just 40 miles from Twin Falls. While her body or remains have not been found yet, police believe Berreth was murdered at her Woodland Park home. Frazee was the last person to see the 29-year old flight instructor before she disappeared.

On Dec. 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?’"

In the amended court documents, Kelsey's parents said: "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."

Frazee was later arrested after an investigation and he faced five counts of murder, including two counts of first-degree and three counts of solicitation to commit first-degree murder. He is due back in court Feb. 19.

Earlier this month, Idaho nurse Krystal Jean Kenney, 32, involved in the case was formally charged with one count of tampering with physical evidence. Michelle Stein, a friend of Krystal, told "CBS This Morning" that she may have disposed of the cell phone belonging to Kelsey because she feared for her life.

“She did tell me that she's never seen that kind of evil in anybody... I will just tell you Krystal was very, very scared,” Stein told the morning show about Frazee. “Krystal is a level headed, kind, fun-loving, happy-go-lucky person. She's a tough cowgirl. She was absolutely scared and extremely upset.”

In January, more details emerged about Kelsey and Frazee's relationship after one of her friends, Ashley Cogburn, said the 29-year-old's fiancé was "borderline emotionally abusive."