"I did not kill David Castor," she said in an exclusive interview with ABC News' David Muir. "And I did not try to kill my daughter, period. And I will never say that I did. Ever."
In January, Stacey Castor stood before a judge and jury in the Onondaga County courthouse in Syracuse, N.Y., where she was given the chance to plead her innocence. Jurors listened to the testimony of more than 50 witnesses and deliberated for three days.
She was accused of murdering her second husband, David Castor, by poisoning him with antifreeze in 2005, and attempting to poison her daughter, Ashley Wallace, with a toxic cocktail of crushed pills and vodka. She also was suspected of murdering her first husband, whose grave lies next to David Castor's.
"I think she's diabolical," said assistant district attorney Christine Garvey. "What she has done over the course of seven years is just unimaginable."
Stacey Castor's defense attorney, Chuck Keller, said, "It's a tragedy for everybody. No matter how it comes out ... it's a tragedy. ... There is no happy ending.
Keller hopes the jury will find reasonable doubt.
"They can't take someone's life away when there is a chance that they've got the wrong person," Keller said.
Castor's case was Keller's first murder trial, and he spent almost two years digging up a trail of evidence that, he said, pointed to a different murderer: Stacey Castor's 21-year-old daughter, Ashley.
In court Stacey alleged that her daughter Ashley had murdered both Michael and David.
If Ashley had indeed been responsible for Michael Wallace's death, it would have meant that she killed her biological father by poisoning him with antifreeze at the age of 11.
"It may be easier for people to decide that I did it because I'm the mother," Stacey Castor said. "But kids do things like that. All you have to do is turn on the news or read the paper. There are children that kill children and other people every single day. It happens."
Castor hasn't spoken to either of her daughters since she was arrested at the hospital after Ashley Wallace nearly died in September 2007.
"You know, the truth will come out," Castor said. "I know that the truth is going to come out of this. The evidence will show that I am innocent. And I will go home."
Wallace maintains, along with investigators, that Castor not only killed David Castor, but also killed Michael Wallace, who was Ashley Wallace's father and Stacey Castor's first husband.
Wallace now believes her mother tried to frame her for the deaths of both men.
But Wallace was confused, saying she never had written a note, and she hadn't taken any pills.
"It's just bald-faced evil," District Attorney William Fitzpatrick said. "[Ashley] almost died. She probably came within a couple of doses of Lexapro of dying. No one that I've ever prosecuted was so coldblooded, so heartless as this woman [Castor] was -- and so unemotional about it."
Wallace told ABC News that her mother once was her best friend, but that they have not spoken since the day of Castor's arrest.
"I would've done anything for her," Wallace said. "But she tried to kill me instead."
In 2000, Castor's first husband, the man she called "the love of her life," seemingly passed away from a heart attack. Wallace left Stacey Castor behind with their two daughters, Ashley and Bree Wallace, who is now 18.
The bizarre death of Castor's second husband within five years of the death of her first husband made investigators suspicious.
David Castor's death initially was thought to be a suicide by antifreeze poisoning. Stacey Castor buried him in a grave plot right next to her first husband.
Shortly afterward, investigators exhumed Michael Wallace's body and discovered traces of ethylene glycol, a chemical found in antifreeze.
What's more, forensic lab workers said the exact same chemical was found in David Castor's body.
Police concluded that Wallace also had died from antifreeze poisoning -- and that both men had been murdered.
Wallace's eldest daughter, Ashley Wallace, was distraught.
"I started crying. I got upset," she recalled. "Why would they dig up daddy? Like, he was resting peacefully. I thought it was inhumane, and I didn't like it whatsoever."
Soon, Stacey Castor became the main suspect. Police wiretapped her telephone calls, and in a conversation with close friend Dani Colman detectives heard Stacey Castor's worry and her denial of accusations.
"It scares the living [expletive] out of me because I didn't do this," Stacey Castor said in one of her phone conversations.
Her longtime friend is one of the few people that maintain Castor's innocence.
Colman told ABC News, "Stacey is my best friend. The person that I know wouldn't do this."
Castor's mother, Judie Eaton, also is her staunch supporter.
"I have woke up in the middle of the night and you just lay there and you think, 'How did this happen?'" said Eaton.
Eaton believes her daughter is innocent, and furthermore, she is convinced that her own granddaughter, Ashley Wallace, is responsible for the murders of David Castor and Michael Wallace.
Bree Wallace laments her family's division.
"I feel like [Judie Eaton] took sides and she didn't have to," Bree Wallace said. "She still could have supported my mother and everything. But she didn't have to accuse my sister."
When the verdict was delivered Feb. 4, 2009, Stacey Castor was in disbelief. She was found guilty of second-degree murder and second-degree attempted murder.
"I think honestly that was the best day of my life," Ashley Wallace said. "Because I knew that people knew, I knew that they knew that I didn't do it."
Stacey Castor told ABC News she recalled thinking, "It can't be happening. It's got to be wrong."
She added that she believes she did not receive a fair trial.
"We weren't allowed to present a defense. There were things that were allowed in that shouldn't have been, things that should have been that weren't," she said.
After being asked repeatedly, Stacey Castor did not say what evidence or which witnesses would have altered the result of the trial.
Despite her denials, a judge sentenced Stacey Castor to more than 50 years in prison, the maximum sentence.
"In my 34 years in the criminal justice system, I've seen murderers of every variety and stripe; but I have to say, Mrs. Castor, you are in a class by yourself," Judge Joseph Fahey said at the sentencing. "What you did to David Castor can only be described as premeditated torture.
"Now, as bad and as evil as that is, what you tried to do to your daughter Ashley is simply something that I find I almost can't comprehend," he added. "I've seen a lot of defendants come through this court system, but I've never seen one who was prepared to sacrifice their child to shift the blame away from themselves."
At the sentencing Ashley Wallace said, "I didn't get a chance to say goodbye, and this will be the last time I get a chance. As horrible as it makes me feel, this is goodbye, Mom. As hard as you tried, I survived; and I will survive because now I'm surrounded by people that love me."
It has yet to be determined whether Stacey Castor was responsible for the murder of her first husband, Michael Wallace.
Just this week, prosecutors were back in court to argue the release of the grand jury transcript of the recent murder case against Stacey Castor. If unsealed, the transcript would be used by prosecutors in nearby Cayuga County for its case against her in the death of Michael Wallace.