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DENVER — A man beat his fiancee to death with a baseball bat after failing to persuade a woman he was having an affair with to commit the killing, but later convinced her to help him cover up the crime by cleaning up the bloody crime scene before he burned the victim's remains, investigators said Tuesday.

Dramatic testimony at a Colorado hearing shows the prosecutors' case relies heavily on the cooperation of Krystal Jean Lee Kenney, a 32-year-old Idaho woman who told police that she and Patrick Frazee were having a romantic relationship.

Kenney has pleaded guilty to tampering with evidence and is required to testify against Frazee as part of her plea agreement with prosecutors.

The body of Kelsey Berreth, a 29-year-old flight instructor and the mother of Frazee's infant daughter, has yet to be found.

Tuesday's hearing was held to determine whether Frazee will stand trial. He was arrested on murder and other charges last December, about a month after Berreth was last seen alive on Thanksgiving Day, Nov. 22. Prosecutors announced additional charges this week, including tampering with a deceased body.

Frazee has not entered a plea to any of the allegations.

According to multiple media outlets, testimony at the hearing revealed that police initially found no evidence of foul play inside Berreth's home but later discovered traces of her blood in the bathroom.

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Colorado Bureau of Investigation Agent Gregg Slater testified Tuesday that Kenney told police Frazee had claimed Berreth was abusing the couple's daughter. Slater said there is no evidence that was true.

Kenney said Frazee suggested multiple ways that she could kill Berreth, including poisoning her coffee or striking her in the head with a metal rod and a baseball bat, Slater said. Kenney told police he became angry each time she failed to follow his directions.

Kenney received a call from Frazee on Nov. 22 demanding that she drive to Colorado, Slater said.

"You got a mess to clean up," Frazee said according to Kenney's account.

Kenney told police that she arrived two days later and found a "horrific" scene with blood spattered on the walls and floors of Berreth's townhome, Slater said.

Kenney told police that Frazee had wrapped a sweater around Berreth's head before beating her to death with a baseball bat and stashed her body on a ranch. After she cleaned the house, Kenney said she went with Frazee to retrieve Berreth's body and watched as Frazee burned it along with the baseball bat on his property, Slater said.

She said Frazee later told her he planned to throw the remains in a dump or river.

Kenney took Berreth's phone to Idaho, where she sent text messages to Frazee and Berreth's employer posing as the dead woman then burned the phone, the investigator said.

Kenney told police she wanted to please Frazee and feared that he would harm her or her family if she did not cooperate, Slater said.

The testimony did not reveal prosecutors' theory for a motive for why Frazee would kill Berreth. Her parents argue in a wrongful death lawsuit that they believe Frazee wanted full custody of the couple's 1-year-old daughter. The child has remained with them while the criminal case proceeds.

Investigators initially said Berreth was last seen on surveillance video with the couple's daughter at a grocery store near her home in Woodland Park, a mountain town near Colorado Springs, south of Denver. Police later found footage on a neighbor's surveillance camera showing Berreth, Frazee and their daughter at the entrance of Berreth's townhome later that afternoon.

Prosecutors' evidence also includes cellphone tower data, suggesting Berreth's phone was in the possession of either Frazee or Kenney after Nov. 22, the date Frazee told police he last saw Berreth.

Frazee, 32, has been jailed since his arrest.