The fiancé of a Colorado mother who has been missing since Thanksgiving and is believed to be dead pleaded not guilty to murder charges Friday.

Patrick Frazee, 32, was arrested in December in connection with Kelsey Berreth's disappearance. Prosecutors allege Frazee beat Berreth to death with a baseball bat after failing to persuade the woman he was having an affair with to carry out the killing.

The mistress, Krystal Jean Lee Kenney, allegedly helped Frazee clean up the scene after the murder before Frazee burned Berreth's body, prosecutors said.

Kelsey Berreth was reported missing by her mother in Woodland Park, Colorado, on Dec. 2, 2018. Woodland Park Police Department

Berreth, 29, was last seen on a neighbor's surveillance camera with her year-old daughter, Kaylee, and Frazee outside her Woodland Park home Nov. 22. Earlier that day, she was seen shopping with her daughter at a local supermarket.

Berreth, however, wasn't reported missing until Dec. 2 by her mother.

Kenney told police that Frazee planned to dump Berreth's remains in a river or a landfill. The body still has not been found.

Kenney pleaded guilty in February to tampering with evidence and agreed to testify against Frazee as part of a plea deal.

Frazee, who is Kaylee's father, was charged in December with two counts of murder and three counts of solicitation to commit murder. Prosecutors in February announced additional charges including tampering with a deceased body.

His trial is set to begin in October.