April 22, 2009 -- Stacey Castor wanted people to believe that she was simply an unlucky wife after her first husband died of an apparent heart attack in 2000 and her second of an apparent suicide in 2005 by poisoning himself with anti-freeze.
But sympathy for Stacey Castor turned to suspicion after the exhumation of her first husband Michael Wallace's body proved that, like David Castor, he too died of anti-freeze poisoning. And in a bizarre twist, Castor eventually also became a suspect in the attempted murder of her own 20-year-old daughter, Ashley Wallace.
"She was my best friend," Ashley Wallace told ABC News' David Muir in an exclusive interview. "She was, and then she took that all away. I would've done anything for her. But she tried to kill me instead."
Soon after the September 2007 discovery that both of Stacey Castor's husbands had been poisoned, detectives Dominick Spinelli and Valerie Brogan of the Onondaga County Sheriff's Department paid a surprise visit to Stacey's residence in the small town of Clay, N.Y., outside Syracuse.
She was taken to police headquarters and questioned about the deaths of her husbands, adamantly defending herself with her version of events and insisting that on the day David Castor allegedly committed suicide, she repeatedly called their home to check on him. But when Spinelli told Stacey that phone records showed she had in fact called only once, the three-hour interrogation abruptly came to an end.
"She was getting anxious, and she shut down and said, 'That's it. I'm done. I want an attorney,'" Brogan recalled.
At that point, detectives believed that Stacey Castor, now 41, panicked and searched for an escape.
Ashley Wallace, Stacey's older daughter from her first marriage to Michael Wallace, said she was outraged and shocked to learn that her mother and best friend was a murder suspect, and said she even told her mother something that would later come back to haunt her.
"I wish that I had done this so they would take the focus off you and it would put the focus on me," she recalled saying.
As detectives listened to Stacey's wiretapped phone calls in the days following her interrogation, two detectives showed up on Ashley's first day of college in September 2007 to tell her that police had exhumed her father's body and discovered that he had not died of a heart attack -- he had been murdered.
"I started crying," she recalled. "I got upset. Why would they dig up Daddy? He was resting peacefully. I thought it was inhumane, and I didn't like it whatsoever."
911 Call, Suicide Note Complicate Case
Hysterical, Ashley called her mother to tell her that detectives had come to her school. When Stacey picked up Ashley at school, she made an unusual proposal.
Ashley said her mother suggested, "Oh, we've had a hard week, let's just drink."
For a 20-year-old girl, it seemed like an irresistible idea. "What kind of teenager wouldn't think that was awesome," she said. "Your parents just gave you permission to drink. Sweet. So I drank with her."
Stacey bought a six-pack of Watermelon Smirnoff Ice, and handed her daughter one in a glass. Ashley drank it and then proceeded to drink straight from the bottles, but she soon became violently ill. She said her mother then gave her a pill to sleep off the alcohol, and she went to bed.
Then next morning Ashley went to school. But when she came home, she said that her mother was waiting with another offer of alcohol.
Stacey handed Ashley a cup with vodka, Sprite and orange juice mixed together. Ashley said the drink tasted bitter and awful, but she drank it anyway. After Stacey and Ashley finished drinking, Ashleywent to sleep.
The next morning, Stacey's younger daughter, Bree Wallace, was concerned her sister had not come out of her room and went to check on her.
"Her mouth was open and her eyes were wide open and they were all glassy," Bree Wallace, 17, recalled. "I tried to call her name and she didn't answer me. I screamed for my mom and she came flying."
For the third time in eight years, Stacey Castor called 911.
"I need an ambulance," she said on the 911 call. "My daughter, I believe, had taken some pills. Ashley? She's having trouble I think. Ashley? Ashley? Oh my God, oh my God."
Stacey told the 911 dispatcher that Ashley had swallowed dozens of pills and a bottle of vodka. When Bree Wallace went back into her sister's room, she discovered something that turned the case on its head: an apparent suicide note. CLICK HERE to read the note.
The 750-word typewritten letter was a shocking confession to the murders of Michael Wallace and David Castor, Ashley Wallace's father and stepfather. The note -- with Ashley's name at the bottom -- gave a detailed account of the poisonings only the murderer could know. In the note, Ashley also allegedly said goodbye to her boyfriend and sister and offered this message to her mother: "Now everyone is going to know what really happened, and they know it wasn't you, it was me." The final words: "Please don't hate me. Remember, I love you."
Ashley Wallace was rushed to the hospital, where Sgt. Michael Norton was the first to see her arrive.
"They wheeled her in," he said. "She was basically unresponsive. They were running around, trying to save this girl's life."
According to Ashley Wallace's emergency room doctor, Daniel Olsson, "Her heart was beating at 170, 190 times a minute and was very erratic," and she was not making sense.
"If they didn't figure out what she had ingested or how much, there was a real potential she was not going to make it," Norton said.
Stacey Castor's Scapegoat: Her Daughter
Bree Wallace was at the hospital with Stacey, who was initially not allowed into her daughter's room.
"She was shaking and saying, 'I don't know why they won't let me see her,'" Bree Wallace said. '"I'm her mother. I should be in there.'"
When she was eventually let in to see her daughter, "I told her that I loved her," Stacey said. "It was the last thing I got to say to her."
Ashley recalled waking up in the hospital after her overdose.
"I couldn't really see," she said. "It was blurry. I feel like I'm in tunnel vision. I realize I'm in the hospital because my arms are tied down. I had ripped a tube out from thrashing around."
Ashley recounted her conversation with Norton, the first detective to speak with her. "He's yelling at me, 'How many pills did you take? What did you take? What did you drink?"
When Norton told her there was a suicide note, he said Ashley looked at him as if he were "out of his mind."
"I'm like, 'What are they talking about?'" she said. "'I didn't do any of these things that you're saying that I did.'"
Investigators faced a perplexing question: Did Ashley write the suicide note and kill her two fathers? Or could it be possible that Stacey had killed her two husbands and now tried to murder her own daughter in attempt to frame her?
Lead detective Spinelli believed Stacey Castor was responsible. "Stacey knew our grip was getting a lot tighter on her. There was really no way out of it besides using her daughter as a scapegoat."
Stacey was arrested at the hospital and charged with the murder of her second husband and the attempted murder of her daughter.
"I would die for my kids," Stacey said. "I could never hurt them like that. Ever. My kids were my life. I could never do that to Ashley."
Mother vs. Daughter in Murder Case
Prosecutors believed Stacey Castor was guilty, but she maintained her innocence.
"How can you sit there and blame your own child?" Ashley asked. "That's what I just don't get. I didn't try to kill anybody, and I would never kill anybody."
But Ashley's grandmother, Judie Eaton, believes in daughter Stacey Castor's innocence.
"I have woke up in the middle of the night, and you just lay there and you think, How did this happen?" she said. "She doesn't deserve this."
Eaton also indicates that Ashley had "ample time and opportunity" to have committed the murders.
Soon mother and daughter faced off in court, and "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.