By Holly Yan, CNN

(CNN) -- It's been almost three months since Kelsey Berreth's toddler has seen her mom. Three months since anyone has seen the Colorado mother alive.

During that time, authorities charged Berreth's fiancé, Patrick Frazee, with murder -- even though the body has not been found.

Now, Berreth's parents have updated a wrongful death lawsuit against Frazee, saying a 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 [their daughter] KB and/or Kelsey to leave KB with him and Kelsey would not agree," said the amendment, filed in a US district court on Friday.

The update was made to a January wrongful death lawsuit filed by Cheryl-Lee Ellen Berreth and Darrell Lynn Berreth.

The Berreths have been searching for answers since Kelsey vanished last Thanksgiving.

The parents grew concerned after the 29-year-old didn't respond to calls or texts for a week. So Cheryl-Lee Berreth called Frazee on December 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.

According to the document, Frazee told Cheryl-Lee Berreth that he and Kelsey broke up on Thanksgiving, and that "Kelsey agreed to share with Frazee 50/50 custody of their daughter."

When pressed for more details about Kelsey's disappearance, "Frazee suggested that Kelsey may have flown somewhere with a friend or co-worker," the amended complaint states.

Attorney Ed Farry, who's representing Frazee in the civil case, declined to respond to allegations in the amended complaint "because any such response will be made through a court filing," he said.

The Berreths are asking for a jury trial in the wrongful death lawsuit. The case was filed in a federal civil court because Frazee lives in Colorado, but the plaintiffs live in Idaho.

More twists, few answers

As the civil case moves forward, the criminal cases involving Frazee and a woman from Idaho have taken some unexpected turns.

Frazee faces two counts of first-degree murder and three counts of solicitation to commit murder in the first degree.

Prosecutors filed two murder charges because they have different theories. One is that he acted alone to kill Berreth; the other is that he -- either alone or with other people -- killed her during a robbery.

The solicitation charges refer to three incidents in which someone was solicited to commit the crime and could mean Frazee is accused of soliciting the same person three times or soliciting three people on different occasions.

Frazee will have a preliminary hearing Tuesday. Adam Steigerwald, the public defender representing Frazee in the criminal case, has not returned CNN's requests for comment.

In a related case, Idaho nurse Krystal Lee Kenney told a judge on Friday that she threw away Berreth's phone after Frazee committed a homicide.

"I learned Patrick Frazee had committed a homicide on approximately November 22, 2018," Kenney said in a statement read in court.

"I knew that law enforcement would be investigating that crime. I moved the victim's cell phone, with the intent to impair the phone's availability in the investigation."

The Berreth family cited Kenney's statement Friday in their updated civil lawsuit against Frazee.

What about Kaylee?

As adults prepare to battle in multiple court cases, a 1-year-old is missing both her mother and father.

Kaylee, the couple's daughter, will keep living with Kelsey Berreth's parents for now, a judge ruled February 7.

Kaylee technically remains in the legal custody of the Teller County Department of Human Services, though Berreth's parents have been taking care of the girl since December.

But court records show Frazee's mother tried to intervene.

The judge scheduled an April 4 hearing to determine the next steps in Kaylee's custody case.