New York woman’s poisoning by son’s ex-girlfriend was ‘an act of vengeance’ during ‘toxic’ relationship: doc

EXCLUSIVE: Dr. Mary Yoder was the picture of health when the wife and mother suddenly became violently ill.

It was 2015 and the 60-year-old chiropractor, who ran a thriving upstate New York practice with her husband Dr. William Yoder, suspected she may have had a stomach bug. After spending the night vomiting, Yoder went to the hospital where she was expected to stay overnight. However, by the time her three children headed to Yoder’s side, she went into cardiac arrest several times. 

The beloved grandmother suffered an excruciating and sudden death.

The perplexing and shocking case is featured in Oxygen’s true-crime docu-series "Killer Motive," which returned for its second season in January. It features interviews with investigators, those who knew Yoder, as well as prosecutor Laurie Lisi.

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In the special, friends described Yoder as an active member of the community who kept busy with pottery, dancing, and gardening. She was seen as "very positive and very full of life." 

"When we first spoke to the family, they were absolutely devastated," Lisi told Fox News. "But they were also absolutely perplexed. How could someone who exercised daily become so sick and die out of nowhere? They immediately wanted an autopsy done because they were frustrated. Even the doctors had a hard time figuring out what exactly happened. It took several specialists to evaluate Mary. They wanted answers."

Tests showed Yoder was poisoned. One expert suspected colchicine, a drug used to treat gout, an inflammation of the joints. It can be fatal if administered incorrectly.

However, Yoder wasn’t diagnosed with the painful, disabling form of arthritis.

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At first, William raised red flags when it was discovered he had developed a relationship with his widowed sister-in-law, the documentary revealed. However, the grieving widower maintained his innocence and was eventually cleared.

Eyes soon turned to Yoder’s son Adam. According to the documentary, an email account in his name was used to order the poison. And then an anonymous letter sent to the medical examiner’s office and the local sheriff’s department insisted Adam, who allegedly had fits of rage, was to blame.

"My impression of Adam was that maybe he had some depression issues, but he was extremely smart," said Lisi. "And he was incredibly devoted to his mother. They had a very strong bond. To be honest, there was no reason to look at him until that letter came in."

According to "Killer Motive," Adam was smoking a cigarette when his car was searched by investigators. When a bottle of colchicine was found in Adam's vehicle, the cigarette "almost fell out of his mouth."

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But after investigators dug deeper into Adam’s whereabouts at the time of his mother’s poisoning, it turned out he had gone to Long Island five days prior to visit one of his sisters. The siblings were together until their father called them about Yoder’s health, which quickly took a downward spiral.

The next suspect was Kaitlyn Conley, Adam’s girlfriend who also worked at the family’s clinic.

"I would describe their relationship as a very toxic relationship," Lisi explained. "They were always on-again, off-again. When you look back at their relationship, it’s clear to see that whenever they broke up, Kaitlyn would do certain things to get Adam Yoder to come back to her."

"She made a false allegation of a rape charge against him," Lisi continued. "She told him that she had experienced an ectopic pregnancy, had almost bled to death and the child was his. At the time, he was briefly involved with someone else. But as soon as she provided this information to him, he immediately broke it off with the other girl and went back to Kaitlyn. It was just an extremely toxic relationship."

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Investigators also discovered that in 2015, Conley gave Adam supplements that she claimed would help him focus on his final exams for college. He not only became sick but experienced similar symptoms to what his mother had. 

"He had no idea what was wrong with him nor did the hospital," said Lisi. "Basically they chalked it up to some kind of a bad stomach virus. So he sort of had in the back of his mind that maybe she tried to poison him. But when he confronted her with the allegation, she became very upset. And then Adam felt guilty for even making such a serious allegation. So he dropped it."

It is believed the Yoder family didn’t suspect what was really going on with the couple behind closed doors. Not only did they welcome Conley as part of the family, but Yoder had a warm friendship with her son’s girlfriend.

Conley admitted to police she wrote the anonymous letter, claiming she was afraid of Adam, who was allegedly abusive. 

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Forensic evidence would prove that Conley set up the email account that was used to order the colchicine, Fort Worth Star-Telegram reported. Not only had Conley searched for poisons online but prepaid gift cards were used to purchase the colchicine. Her DNA was also found in the colchicine vial.

"I think Kaitlyn wanted vengeance from Adam for breaking things off," said Lisi. "So she decided not only she would take his mother’s life, but that she would pin the homicide on him. I think that’s why she took the risk of writing that anonymous letter - because of her desire to seek vengeance on Adam Yoder."

"The evidence showed that during lunchtime, Mary went out to run an errand," Lisi shared. "That gave the office manager access to the kitchen area, which is where Mary Yoder kept her almond milk and protein powder shake that she would normally have for lunch every day.

"At some point, Kaitlyn Conley, who was the office manager at the time and alone, took out the colchicine she had purchased months prior, in powder form, and put it in the protein powder or maybe the milk. Mary came back at lunchtime, prepared the drink for herself and drank it."

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While Conley’s first trial ended in a hung jury, the-then-24-year-old was convicted of manslaughter in 2017. She was sentenced to 23 years in prison. 

The documentary showed that at the sentencing, Adam tearfully expressed his regret for bringing Conley into his family’s life.

"As much as I hate her, which is more than I ever thought I would be capable of hating anyone, I hate myself infinitely more," he admitted.

Lisi said it’s difficult to say whether Conley would have continued her quest in getting back at Adam if his mother had survived. However, she said there’s no denying her agenda.

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"When Mary Yoder died, it did what Kaitlyn Conley wanted," she said. "Adam Yoder re-introduced Kaitlyn back into his life, at least for the following three months after Mary died. So if her goal was to get back with him, it was successful, although briefly. If Mary had survived and Kaitlyn had been there to help her, I think that would have worked too in getting back into Adam’s life."

"But if Adam had left her again, based on everything, I would say she probably would have done something," said Lisi. "I could see her doing something again to get his attention… You don’t always get what you see. Kaitlin Conley put out a certain image but it wasn’t really who she was."

"Killer Motive" airs Saturdays at 6 p.m. ET on Oxygen. The Associated Press contributed to this report.