"Her remains have not been found, nor has any evidence relating to her death," the Woodland Park Police Department said in a statement.
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Berreth, 29, was last seen on a neighbor's surveillance camera with her year-old daughter, Kaylee, and her fiancé, Patrick Frazee, outside her home Nov. 22, police said. Earlier that day, she was seen shopping with her daughter at a supermarket in Woodland Park, Colorado.
She wasn't reported missing until Dec. 2 by her mother.
Investigators say that after the murder, Frazee convinced his mistress, Krystal Jean Lee Kenney, to clean the crime scene before he burned Berreth's body. Kenney told police that Frazee planned to dump the remains in a landfill or a river.
Kenney has pleaded guilty to tampering with evidence and has to testify against Frazee as part of a plea deal. Frazee has not yet entered a plea.
Woodland Park Police Chief Miles De Young said Wednesday that not locating evidence or Berreth's remains at the landfill was not the "outcome that we hoped for."
Police will continue to investigate and are "committed to bringing justice to Kelsey's family," the department said.
Minyvonne Burke is a breaking news reporter for NBC News.