The jury of six men and six women deliberated for less than four hours before reaching a verdict.
Frazee, 33, was found guilty on all charges: two counts of murder, three counts of solicitation to commit murder and tampering with a deceased human body. He had denied killing Berreth, 29, and pleaded not guilty. The jury trial began Nov. 1.
A judge sentenced Frazee to life without parole for first-degree murder plus an additional 156 years.
Beth Reid, one of the prosecutors on the case, said after the verdict that she never witnessed Frazee express anything other than the emotionless state he exhibited in court.
"I think based on my observations of Patrick Frazee over the last year, that there’s a potential that Patrick Frazee indicates and is consistent with a sociopath," Reid said. "And that’s a person who has basically an inability to feel empathy with other people."
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In closing arguments, prosecutors argued that Frazee had been planning Berreth's death for months, while the defense called evidence presented during the trial "circumstantial" and "made up."
Berreth was last seen publicly Thanksgiving Day last year. Surveillance video shows her shopping with Kaylee, the couple's year-old daughter, at a Safeway grocery store in her hometown of Woodland Park on Nov. 22, 2018.
Frazee, who did not live with Berreth, has said he last saw her that day and last heard from her Nov. 25, but she was not reported missing until Dec. 2 by her mother. Berreth's body has not been found.
Berreth's mother, Cheryl Berreth, spoke to the court prior to the sentencing to request Frazee get the maximum sentence and the removal of his parental rights, as the family intends to adopt Kaylee.
She said she cleaned the mess but left small spots for investigators to find.
Frazee later told her he beat Berreth to death with a baseball bat, and then he burned her body at his ranch, she testified. "He just said he swung away, and that it was really hard," Lee said, adding that Frazee told her Berreth begged him to "please, stop."
Prosecutors said Kaylee was in the apartment while her father killed her mother.
Lee said she was directed by Frazee to bring Berreth's cellphone to Idaho so it would look like she left. She complied, and burned the phone. It last pinged Nov. 25 near Gooding, Idaho, investigators have said.
Lee was charged with tampering with evidence and pleaded guilty in February, agreeing to testify against Frazee in a plea deal. Prosecutors said that before Frazee killed Berreth, he tried three times to convince Lee to commit the crime.
An inmate testified during the trial that when he and Frazee were in the same jail, Frazee asked him to kill Lee. Notes between the two list more than 10 other people who needed to “disappear or be unseen" until after the trial, a law enforcement official testified Friday.
A friend of Frazee's, Joe Moore, testified that the two were together in April 2018 when he asked Frazee how things were going with Berreth. “I figured out a way to kill her," Moore said Frazee replied. During the conversation, Frazee grinned and said "No body, no crime, right?" Moore testified.
Following Berreth's disappearance, the two met up again Dec. 20, and Frazee expressed surprise in the media's interest in the case. Frazee was arrested and charged with murder the next day.
Elisha Fieldstadt is a breaking news reporter for NBC News.
Doha Madani is a breaking news reporter for NBC News.