Woman Fatally Poisoned Former Boyfriend’s Mom To Get Toxic Revenge On Her Ex

On the morning of July 20, 2015, Mary Yoder, 60, a chiropractor with a thriving upstate New York practice she ran with her husband, William Yoder, was the picture of health. 

A friend who’d seen her early that day recalled Mary radiated her signature warmth and inner peace. But after lunch time, things changed. Mary felt sick and struggled to finish her scheduled appointments. 

That evening, Mary told her husband she believed she had a stomach bug. The next morning, she went to the hospital, where doctors expected her to be released after an overnight stay. 

Instead, Mary’s condition “spiraled downward,” Oneida County Asst. D.A. Laurie Lisi told “Killer Motive,” airing Saturdays at 6/5c on Oxygen.

William Yoder called their three kids and summoned them to the hospital, where Mary went into cardiac arrest. Her death was painful and sudden.

“What the hell happened?” That was the question asked by Sharon Groah, Mary’s longtime friend and client, as well as others in the tight-knit community of Whitesboro, New York.

Kaitlyn Conley

An autopsy offered little in the way of answers, and a closer look suggested Mary was poisoned. Tests were ordered to search for the presence of toxins including arsenic, cyanide, and other standard poisons. After a number of weeks, the results came back negative. 

A poison control expert suspected Colchicine, an anti-gout drug with a narrow therapeutic index. That means that “the range between therapeutic and toxic doses is small, and in some cases they overlap.”

In its pure form, Lisi told producers, a tiny amount “is deadly.”

In October 2015, lab results revealed that Mary Yoder, who did not have gout, had a lethal amount of Colchicine in her system.

Had she consumed something that had accidentally been poisoned? Or had she intentionally been dosed with the drug? Investigators determined it was the latter. 

Detectives began their investigation with William, who’d met Mary in college in 1975. They looked into the victim’s life insurance policies as potential motive. The search revealed nothing suspicious and the prospect of an insurance payout as a motive was ruled out. 

William’s burgeoning relationship with a widowed sister of his wife also initially raised a red flag, but investigators eventually determined that they needed to pursue other avenues.

Then, in November 2015, the case took an abrupt turn thanks to an anonymous letter mailed to the Oneida County Sheriff’s Office that pointed the finger at Adam Yoder, William and Mary’s son.

Mary Yoder

The missive mentioned Colchicine and said that a container of it could be found under the passenger seat of Adam’s Jeep.

Investigators considered possible motives for Adam to murder his mother. One possible reason: a monetary inheritance. 

In December 2015, Adam was brought in for questioning and denied culpability for his mother’s death. Investigators asked him if they could search his vehicle, and after consulting with a lawyer, Adam gave them his OK. 

Oneida County Sheriff’s Office investigator Mark Van Namee told “Killer Motive” host Troy Roberts that Adam was smoking a cigarette when his car was searched, and when the bottle of Colchicine was found, “it almost fell out of his mouth.”

Investigators dug deeper into Adam’s whereabouts at the time of his mother’s poisoning. Five days prior he’d gone to Long Island to visit one of his sisters. He stayed there until his father called about his mom’s dire health situation.

Was the author of the anonymous letter framing Adam? Who would do that? William Yoder’s name rose to the top of the list of persons of interest. But then detectives spoke with Adam’s on-off girlfriend, Kaitlyn Conley, who worked at the Yoders' chiropractic clinic. 

During an interview with police she admitted to writing the letter and claimed that she was afraid of Adam. Conley’s story raised red flags, though, in light of Adam’s sturdy alibi.

But investigators lacked a clear connection between Conley and the case. They also struggled to find a motive. Mary was Conley’s boss and a friend. 

However, when investigators researched Conley’s background they learned that she and Adam had a fraught and at times toxic on-and-off relationship. They also discovered that in April 2015 Conley had given him supplements intended to help him focus attention on his studies that ended up making him sick, the Utica-Observer Dispatch reported in 2017.

In February 2016, detectives turned to computer forensic experts for help. This line of investigation turned up emails as well as prepaid gift cards used in the purchase of the Colchicine that led to Conley. Her DNA was also found on the Colchicine vial found in Adam’s Jeep. 

Why would she harm Mary Yoder?

“Adam Yoder was the motive,” said Lisi, adding that one way to exact revenge on her ex-boyfriend was to take away someone he loved. 

Investigators theorized that Conley spiked a protein drink Mary drank each day. Working at the clinic gave her access.

In May 2016, Conley, 24, was charged with second-degree murder. 

Her trial began in April 2017. Prosecutors believed they had a strong case, including Conley's admission of writing the anonymous letter, her DNA on the Colchicine bottle, and the emails and debit cards used to order and pay for the drug.

They also knew, though, that a jury might not think that Conley looked like a killer. The jury ended up deadlocked and the judge declared a mistrial. 

As prosecutors prepared for a second trial, Adam informed them that Conley had plugged her phone into his computer around the time his mother died. 

Forensic experts searched his laptop and discovered that a backup of Conley’s phone had been created. It revealed that she had searched the internet about the most lethal poisons. 

The invisible evidence added heft to the prosecution’s case at the second trial. On November 6, 2017 Conley was found guilty of first-degree manslaughter.

Adam Yoder spoke during the hearing, saying, “I hate the defendant with every bone in my body and every drop of blood in my veins.”

Conley was sentenced to 23 years in prison. 

To learn more about the case, watch “Killer Motive,” airing Saturdays at 6/5c on Oxygen, or stream episodes on Oxygen.com.