Jonathan Hearn, the prosecution's key witness in the murder trial of Sabrina Limon, testified Thursday in explicit detail how he and Limon, his former lover, plotted the killing of her husband.

The 27-year-old Hearn told the jury how he and Limon first met at a Victorville Costco in 2012 and began an affair within months. Soon they were professing their love for each other and, even after her husband discovered their relationship, planning for a future together.

More than a year into their relationship, they began to joke about killing her husband, he testified.

"By the end of 2013, early 2014, it definitely turned from humor to something that actually began to materialize," he said. 

Hearn, his voice steady, his tone matter of fact, testified for nearly three hours Thursday afternoon as to how they began planning the husband's death. Sabrina Limon shook her head at times during his testimony. 

He confessed earlier this year to fatally shooting 37-year-old Robert Limon at a Tehachapi railway yard on Aug. 17, 2014. Prosecutors offered him a prison sentence of 25 years and four months in prison in exchange for his "honest, truthful" testimony.

Sabrina Limon, 37, faces life in prison if convicted. 

Her attorney, Richard Terry, has argued Hearn, an admitted killer, is the only evidence connecting Sabrina Limon to the killing. He has said Hearn's word cannot be trusted. 

Hearn, a former Redlands firefighter, told jurors he and Sabrina Limon first met in August 2012 when he saw her at Costco, where she handed out samples. Hearn had been tasked with buying food for the fire station, and the two talked together.

He said he saw Sabrina Limon on repeated trips to Costco, was attracted to her and noticed she didn't wear a wedding ring. He asked for her number, and she gave it to him.

She told him she was married soon after they began calling each other, Hearn testified. They decided to only remain friends, but Hearn, who had never had a steady girlfriend, said they continued flirting and his feelings for her deepened.

Hearn described Sabrina Limon as having a "magnetic personality," and as loving and caring. They kissed in November 2012, and later had a discussion about the future of their relationship.

At the time, Hearn testified, he didn't "see it leading to anything good." Nevertheless, by the end of 2012 they'd began a "full-fledged affair," he said.

They met either at his place in Hesperia, or her house in Helendale when her husband and two children were out, Hearn testified. 

Prosecutor Eric Smith displayed some of the photos in court that Hearn testified Sabrina Limon had sent him from her cellphone. In some, she puckered her lips for the camera; others showed her in various states of undress.

Hearn testified he cared for Sabrina Limon, and their relationship was not solely based on sex.

"I would say by early 2013 it was primarily an emotional affair," he said. 

In March or April 2013, Hearn testified, Robert Limon found out about the affair and told him to stay away from his wife.

Hearn, who spoke of conflicting feelings about what he'd done given his Christian schooling, said he made an effort to stop the relationship. He wrote himself letters about seeking forgiveness for meddling in the Limon's marriage.

"I am blessed, but torn," he wrote in a letter displayed for the jury. "I cannot afford to hurt Robert again. Yet I love Sabrina."

He also wrote, "Ultimately, I want this woman to know that she is amazing and loved by God and me. But I cannot also disrespect, hurt or dishonor her incredible husband."

In a symbolic gesture, Hearn testified, he wrote a personal check to be payable to "Jesus, the one who helped stop me, tried me and showed me Grace." The check was also made out to Robert Limon, as a sign that Hearn had sought and been granted forgiveness, he said. 

Despite his conflicting emotions, the affair soon started back up, Hearn testified. And by late 2013, he said, Sabrina Limon, who up to that point never complained about her husband, began to regularly disparage him.

The Limons were involved in an open marriage. Hearn testified Sabrina Limon said her husband objectified her and just "passed her off" to other guys to engage in sex acts with her. She wasn't made to feel special.

After a number of these discussions, Hearn said, their conversations began to veer toward an actual plot to kill Robert Limon so they could eventually marry and live out the rest of their lives together. 

They considered a number of methods, Hearn said. He first thought of staging a car accident, but abandoned that idea due to how much investigation it would incur. Another option, arson, was also dropped.

Poisoning, however, seemed like a viable option, and he and Sabrina Limon decided they would kill Robert Limon by poisoning his favorite dessert: banana pudding with Nilla wafers. 

"I did plan the poisoning of Robert Limon with some necessary assistance from Sabrina Limon," Hearn told the court. 

By using a prepaid credit card and an alias, Hearn purchased arsenic trioxide online, he testified. He said he researched the amount necessary to kill a human being, and experimented by placing some of the arsenic on a "piece of meat" and feeding it to a neighborhood dog "who caused me some considerable issues in the past."

In court documents, Hearn is quoted as saying he no longer saw the neighborhood dog. 

Hearn testified he made banana pudding and laced it with arsenic trioxide. He said he placed it in a small Tupperware container and gave it to Sabrina Limon to give to her husband to take to work as part of his lunch.

Robert Limon, a Burlington Northern Santa Fe Railway employee, usually worked in Barstow. Sometimes he filled in for other workers at a BNSF shop in Tehachapi.

Hearn said they planned the poisoning specifically on a day when Robert Limon was working in Tehachapi because they didn't believe the hospital there had as many resources as did hospitals in metro areas. He said they believed the poisoning would go undetected.

But on the day Sabrina Limon gave her husband the poison, Hearn testified, they discussed the possibility of getting caught due to how many calls and messages they'd made and sent to each other. They decided to abort the plan, Hearn said.

He said he believes Sabrina Limon called her husband and told him the bananas had gone bad and not to eat the pudding.

Shortly afterward, Hearn testified, he bought Sabrina Limon a "burner" phone with no registration information, believing it would make it more difficult for law enforcement to track their interactions. He said they decided to wait a few months before making another attempt on Robert Limon's life.

He has not yet told the jury about Robert Limon's shooting.

A number of Google searches were found on Hearn's computer, according to court testimony. Among them were "arsenic," "arsenic trioxide" and different websites with recipes for banana pudding. Hearn admitted in court to making those searches.

He also admitted to asking the following question on WikiAnswers: "How soon after a person dies does a life insurance policy pay out?" 

Sabrina Limon collected $300,000 in life insurance after Robert Limon's death, according to court testimony. 

Hearn's testimony resumes 9 a.m. Friday. 

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