The Colorado man accused of killing the mother of his child last year plans to blame the murder on his former girlfriend, who is a key witness against him, according to court documents filed by defense lawyers.

Patrick Frazee, charged with the slaying of fiancé Kelsey Berreth, is set to implicate former girlfriend Krystal Lee, his lawyer wrote in court papers outlining potential defense witnesses and strategy.

A vigil for Kelsey Berreth in 2018. Kelsey Brunner / The Gazette via AP

The defense might introduce "evidence that other individuals committed or fabricated portions of the crimes alleged against Mr. Frazee, to include Krystal Lee," according to the memo filed by his lawyer, Payne Steigerwald, in Teller County.

Berreth, a 29-year-old flight instructor and the mother of Frazee's infant daughter, was last seen alive on Thanksgiving Day last year.

Frazee's trial is set to begin Oct. 28 in Cripple Creek, Colorado.

Lee, an Idaho nurse who also goes by the name Krystal Jean Lee Kenney, is set to testify that Frazee beat Berreth to death with a baseball bat before he burned the victim's remains, investigators have said.

The nurse has admitted to helping Frazee cover up Berreth's slaying by taking the victim's cell phone to Idaho to throw off investigators, authorities said.

She's pleaded guilty to tampering with evidence and must testify against Frazee as part of that deal with prosecutors.

"I learned that Patrick Frazee had committed a homicide on approximately Nov. 22 in Teller County," she said in a statement released by prosecutors after reaching the plea agreement.

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

During a pretrial hearing in February, a Colorado Bureau of Investigation agent testified that Frazee had suggested multiple ways that Lee could kill Berreth, including poisoning her coffee or striking her in the head with a metal rod and a baseball bat.

The girlfriend refused to kill Berreth, authorities said, but later agreed to help Frazee clean up the crime scene.

Defense lawyers missed a Monday night deadline to tell the judge it would present evidence of an alternate suspect. But there's still space, under Colorado statues, for that evidence to be permitted at trial, according to criminal defense lawyer Scott Robinson.

Frazee's defense needs to show Lee had motive and opportunity.

"His defense will have no problem proving that for Krystal Lee," said Robinson, legal analyst for NBC affiliate KUSA. "She has admitted her involvement, and cell phone records place her at the scene."