On Thanksgiving Day 2018, Colorado mom Kelsey Berreth vanished without a trace after a trip to the grocery store, where she was last seen in surveillance footage pushing a cart with her 1-year-old baby in tow.

Four weeks later, her cattle rancher fiancé was arrested for her murder, with authorities alleging he’d concocted several schemes to get rid of his wife-to-be—and enlisted the help of his secret girlfriend.

Patrick Frazee, 33, has pleaded not guilty to several charges related to the 29-year-old’s disappearance, including two counts of first-degree murder and three counts of solicitation to commit first-degree murder.

Nearly a year after the tragedy first unfolded, opening arguments in Frazee’s murder trial are set to start this week in a Teller County courtroom, where prosecutors plan to argue he tried to persuade his mistress to help him kill Berreth, before taking matters into his own hands and fatally beating her with a baseball bat on Thanksgiving Day.

Authorities say Frazee then buried the 29-year-old’s body, which has yet to be found, in his backyard.

“My heart aches most for Kelsey’s closest, her parents and brother. Knowing that they have to relive every detail, from worrying about her being missing to finding out the horrific way in which she was murdered,” JoDee Garretson, Berreth’s cousin, told The Daily Beast. “I pray that Patrick Frazee is held accountable so Kelsey’s daughter remains in a safe and loving home with Kelsey’s parents.”

Defense attorneys for Frazee, who insist their client is innocent, plan to argue in court “there may be evidence that other individuals committed or fabricated parts of the crimes alleged,” according to a September court document.

Berreth, a flight instructor at Doss Aviation, was last seen at the Safeway grocery store in Woodland Park, Colo., on Nov. 22, 2018. In security footage, she can be seen placing her daughter, Kaylee, in a shopping cart.

Ten days after the grocery store trip, Berreth’s mother, Cheryl, reported her missing to authorities, stating she was concerned because she had not been able to reach her daughter. In her Dec. 2 statement, Cherly said she had contacted Frazee, who was caring for Kaylee at the time, and he revealed that he hadn’t spoken to his fiancée in days.

When authorities entered Berreth’s home, they found cinnamon rolls in the kitchen that had been set out to cool, and her cars still parked outside. Her cellphone and purse, however, were missing.

Frazee told cops he last saw Berreth on Thanksgiving Day, when she dropped off their daughter at his home . He claimed to have received a text from her three days later.

While it remains unclear why Berreth didn’t spend the holiday with Frazee, her mother previously told NBC the couple did not live together because of the “economy.”

“The relationship has been good. They’re loving,” Cheryl Berreth said, adding that they hadn’t set a wedding date. “It hasn’t been ideal, you know, given the economy and things. They had plans that haven’t worked out as they would have liked. But they’ve dealt with that and they’ve made things work regardless.”

Berreth’s aunt, however, claimed in a Facebook post soon after her disappearance that her niece broke up with Frazee the same day she went missing—a claim that hasn’t been confirmed by police or her family.

Authorities say Doss Aviation received a text from the missing mother’s number on Nov. 25, informing them she would be taking the following week off from work. The same day, her phone last pinged off a cell tower in Gooding, Idaho—nearly 800 miles away from Berreth’s residence in Woodland Park, police said.

Authorities arrested Frazee and charged him with his fiancée’s murder on Dec. 21, after over 75 officers executed an extensive search warrant on the 35-acre ranch where he lives with his mother. At a press conference following the arrest, Woodland Park Police Chief Miles De Young said Frazee murdered Berreth at her townhouse and she is presumed dead.

Prosecutors argued in court documents that Frazee planned to murder his fiancée for two months, making three unsuccessful attempts to kill her, one time with a poisoned Starbucks drink.

Frazee’s girlfriend, Krystal Lee, told authorities that he’d enlisted her help in carrying out his sinister plan, telling her that Berreth abused their toddler and he needed her help. Lee, a nurse from Idaho, said she refused to assist him.

On Nov. 22, Frazee allegedly told Lee that he’d killed his fiancée with a baseball bat after convincing her to put on a blindfold for a “candle smell test,” according to court documents.

Out of fear, Lee said she helped clean up the murder scene at Berreth’s house two days after the incident and watched Frazee burn the body and the baseball bat somewhere in the 29-year-old’s backyard.

Lee pleaded guilty in February to evidence tampering, admitting she helped dispose of Berreth’s cellphone in Idaho at Frazee’s behest. She’s expected to testify against Frazee in accordance with her plea agreement and faces up to three years in prison.

While prosecutors have not yet offered a motive behind Bereth’s murder, her parents argued in a wrongful death lawsuit filed in January that Frazee wanted full custody of the couple’s daughter.

“Upon information and belief, Frazee had motive to kill Kelsey in that he wanted full custody of [their daughter] KB and/or Kelsey to leave KB with him and Kelsey would not agree,” the lawsuit states. The couple’s daughter, Kaylee, is technically in the legal custody of the Teller County Department of Human Services but has been in Berreth’s parents’ care since December.

Prosecutors have declined to seek the death penalty in the case, meaning Frazee faces a maximum sentence of life in prison without the possibility of parole.