During a family dinner on Christmas Eve, Michael Wallace's sister, Rosemary Corbett, recalled: "Mike was coughing a lot" and was "swollen and puffy." His family encouraged him to see a doctor, but Wallace never made it.
In 2000, Ashley, then 11, remembered being at home one day in January with her father.
"He was laying on the couch, making what I thought were funny faces. And all of sudden, he just sticks his arm up in the air and puts his arm on his side and then his arm just fell down," she said.
With her father still on the couch, Ashley left to pick up her sister at school. It would be the last time she'd see him alive.
"I've relived this day over and over again in my head, because what if there was something that I could've done?" she said. "Like, I should've known, but I didn't. I was 11!"
Later at the hospital, doctors told Stacey Castor her husband had died of a heart attack, but Rosemary Corbett, his older sister, was skeptical.
"The color of his skin from head to chest was deep, dark purple. And it was really weird," she said.
Corbett wanted Stacey to have doctors perform an autopsy on her dead brother, but the wife said no.
"When the doctors told me that they believed he'd died of a heart attack, I believed that. There was no reason for me to question that," Stacey said.
No one had reason to question Wallace's death until 2005, when after two years of marriage, Stacey Castor's second husband died under suspicious circumstances. The coroner concluded that David Castor had committed suicide by consuming a lethal dose of anti-freeze. Investigators began to look more closely at the evidence and more closely at the grieving widow, Stacey Castor.
Police said that forensic tests on items seized from inside David Castor's locked bedroom incriminated his wife. Stacey's fingerprints were on a glass half full of anti-freeze, and police found a turkey baster with David's DNA on the tip. For Detective Dominick Spinelli and others from the Onondaga Sheriff's Department, it was all adding up.
Investigators said that David Castor's death was now a homicide, but there was only one way to find out if Stacey Castor was the one responsible for the murder of both David Castor and her former husband, Michael Wallace.
After careful consideration, Spinelli made the unusual decision to exhume Michael Wallace's body.
"The last thing I want to do is disturb someone that's at peace, especially if nothing showed up in his system," said Spinelli.
But confident his "sixth sense" would not fail him, Spinelli watched as the heavy machinery lifted the casket out of the ground.
"What if he's saying, 'It's about time you guys are looking at this, because I didn't just die on my own,'" he recalled thinking.
The exhumation of Wallace's body proved that he, too, had died of anti-freeze poisoning. After a mounting investigation spanning two years, Castor was arrested and convicted of second degree murder in David Castor's death and was also convicted of attempting to murder her daughter Ashley and frame her for her husbands' deaths.
"20/20" was granted full access to all sides of this curious investigation and trial. Stacey Castor talked about the trial and her conviction in an exclusive interview with ABC News' David Muir, who also sat down with the lawyers, detectives, doctors, family and friends who spent a decade watching this mystery unravel.