The murder trial for so-called "cult mom" Lori Vallow began with jury selection Monday, nearly three years since two of her children were found dead and buried on her husband's rural Idaho property in 2020.

Vallow and her husband, Chad Daybell, both religious conspiracists, are accused of killing her children, 7-year-old J.J. Vallow and 17-year-old Tylee Ryan, and Daybell's ex-wife, Tammy, between September and October 2019.

"It was just getting crazier and crazier," Richard Robertson, a private investigator hired to work Vallow's case in 2019, told Fox News Digital of the series of bizarre events that unfolded leading up to her 2020 arrest.

Robertson was initially hired to find Vallow and her late brother, Alex Cox, after they were accused of plotting a drive-by shooting in Arizona in July 2019.

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Once Robertson found Vallow and Cox in Idaho, J.J. Vallow's grandparents asked the private investigator if he had seen any children over the course of his investigation, which is when he began searching for the missing 7-year-old and his sister, who went missing in Idaho in September 2019.

Lori and her late fourth husband, Charles Vallow, had been married since 2006 and adopted J.J. together in 2013 from Charles' relative. Their relationship had been a positive one until 2018, when Lori met Daybell on a podcast discussing theories about the end of the world.

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It was around that time that Charles Vallow noticed a shift in his wife. She was embracing theories about the apocalypse and had started calling her children "zombies," according to investigators. Then, J.J. and Tylee disappeared.

Charles called police to his home in Arizona on Jan. 31, 2019, to report his wife and children missing. In police body camera footage from that evening obtained by FOX 10 Phoenix, Charles can be heard telling police that he thinks his wife has "lost her mind."

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He continues to tell officers that Lori believes she is "a resurrected being of God" and thinks he is a "dark spirit." He says he doesn't "know what she's going to do," with his children, who were with Lori at the time.

"I don't know if she's going to flee with them, if she's going to hurt them," Charles responded, adding that he had spoken to Lori earlier in the day over the phone. He claimed she told him and his church bishop, "Come take the kids. I don't care what happens to them."

Lori went to the police the next day and apparently told authorities she believed Charles was cheating on her. The next month, Lori left her children with Charles and disappeared for 58 days, according to J.J.'s grandmother, Kay Woodcock.

At that point, Charles filed for a divorce and custody of her children. Lori soon began dating Chad Daybell.

In July 2019, Lori's brother, Alex Cox, allegedly shot Charles Vallow to death in Arizona, claiming self-defense. Two months later, in September 2019, Tylee and J.J. went missing for months. Cox died later that year of an apparent blood clot in December 2019.

"As soon as Lori started lying, I had a bad feeling what the outcome was going to be," Robertson said. "We were hopeful that that wasn't what had happened – that she just spirited them off to some compound someplace or something wacko like that. But the longer it went on and the more she dug in their heels, I was sure that something bad really happened. It was only a matter of time."

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J.J. and Tylee were missing for months when police say the couple lied about the children's whereabouts and then slipped away to Hawaii, where they got married in November 2019.

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"At that point, law enforcement was involved," Robertson said. "That's when they put out the flyers about the endangered kids. Multiple agencies got involved. We were tracing Lori and Chad in Hawaii because Lori was using her dead husband, Charles', Amazon account, and we had the login, and we were watching her buy this s--t on Charles' Amazon account and ship it to an address in Hawaii."

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Local agencies from Hawaii, Arizona and Idaho, as well as the FBI, then became involved in the search for J.J. and Tylee, which is when Robertson "stepped back" from his own investigation.

Authorities eventually found J.J. and Tylee's bodies buried on Daybell's property in rural Idaho in June 2020.

Robertson noted that Tylee and J.J.'s cause of death have not been released yet, though he suspects they will be brought up in trial. Investigators found Tylee's remains dismembered and burned while J.J.'s body was wrapped in plastic and tied with duct tape.

Vallow and Daybell allegedly collected J.J.'s and Tylee's Social Security benefits between Oct. 1, 2019, and Jan. 22, 2020, after their murders.

Authorities arrested Vallow in February 2020 and Daybell in June 2020.

The couple was indicted in late May 2021 on multiple counts each of conspiracy to commit first-degree murder and grand theft by deception, first-degree murder and conspiracy to commit first-degree murder related to the deaths of Tylee, J.J. and Tammy Daybell, officials announced at the time. Arizona officials in June 2021 also indicted Vallow in the July 2019 murder of her ex-husband, Charles Vallow.

Daybell has written several apocalyptic novels based loosely on Mormon theology. Both were involved in a group that promotes preparedness for the biblical end times. Vallow and Daybell bonded over their religious beliefs after initially meeting in 2018, when they appeared together for the first time on a podcast discussing theories about the end of the world.

The couple was initially scheduled to stand a joint trial in 2021, but proceedings were delayed due to Vallow's competency examinations and an "exceptionally voluminous" collection of evidence, among other factors. Judge Steven Boyce ruled in February that their cases would be severed because Vallow has refused to waive her right to a speedy trial.