April 23, 2009— -- Just days after the apparent suicide death of her second husband, 48-year-old David Castor, in 2005, Stacey Castor buried him in a rural cemetery in upstate New York. Lying next to him in the neighboring plot was Stacey Castor's first husband, Michael Wallace, who had died five years earlier, at age 38, of an apparent heart attack.
Police would later learn that it was more than coincidence that brought these two men to rest in neighboring graves. When police confirmed that both of Stacey Castor's husbands had died of anti-freeze poisoning, the grieving widow was thrust into the center of a murder investigation.
Stacey Castor, a tall, vivacious redhead, was introduced to Michael Wallace, the man she would call "the love of her life," by friends in 1985. Still a teenager at the time, Stacey said their attraction was instant.
"I knew five minutes after I met him that I was going to marry him," she told ABC News' David Muir in an exclusive interview.
The couple quickly became inseparable, and five years after their first date, they were married at Stacey's parent's home. Judie Eaton, Stacey's mother, recalled that her daughter was "as happy as she had ever been."
For Wallace, a gregarious guy who "liked to be the life of the party," having a good time sometimes meant using drugs and drinking too much, Stacey said.
"He had a problem with both for a long time in his life," she said.
But he also loved Stacey very much, and in 1987 the two welcomed their first daughter, Ashley. It was a life-changing moment for the new mother, who said the two "did everything together."
"I knew from that minute on, my whole reason for being here was to take care of her," she said.
Three years later, Stacey Castor gave birth to another daughter, Bree, and this time, it was her husband's turn to bond. He called their new baby girl his "princess."
Dani Colman, one of Stacey's oldest friends, recalled that Bree was "daddy's little angel" and that "she could do no wrong" in Wallace's eyes.
"There was no talk of any relationship between Ashley and Michael Wallace," Colman said. But Stacey said the connection between father and little sister was not lost on Ashley.
"I think it definitely hurt her," Stacey said.
She said that she made up for the father's slight by spending more time with her older daughter. The two became more than mother and daughter; they became "best friends."
Wallace Falls Ill; Appears 'Swollen and Puffy'
For a while, the family of four seemed normal and happy. Ashley Wallace remembered her childhood fondly.
"We'd just go for a ride in the car, you know? For no reason, just take a ride. That was fun," she told Muir.
And though Stacey and her husband argued from time to time, it was never about money, "because they had none," according to Stacey.
She worked days at an ambulance dispatch company and Wallace worked nights as a mechanic. But over time the relationship started to lose its luster, according to friends. Rumors of infidelity by both husband and wife swirled.
In late 1999, with the holidays approaching, Wallace started to feel very sick.
"It was on and off for like six weeks," his sister-in-law, Melanne Keim, said. Keim recalled Wallace acting like he was "drunk, very unsteady."
But no one could figure out what was wrong with the 38-year-old. Wallace himself told Keim that he was "just really tired of feeling this way."
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.
'Black Widow' Says 'No' to Autopsy
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.
Detectives Exhume Michael Wallace's Body
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.
Go to the "20/20" page at ABCNews.com all week for more on this story and watch the two-hour special Friday on "20/20" at 9. p.m. ET