(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.

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.