"A sixth sense is something you develop throughout your career," he said. "It tells you something isn't right."
Weeks earlier, during their routine interview with Stacey Castor, she had mentioned that her husband may have chosen antifreeze after watching a documentary on Julia Lynn Turner, a woman in Georgia who was convicted of killing her husband and boyfriend with the toxic liquid. There was "something strange about it," and Spinelli couldn't let it go.
Further investigation into Stacey's phone records revealed that she'd only made one call to her husband the day he died.
Police also said that forensic tests on that seized glass, half full of antifreeze, determined that the fingerprints on it belonged to Stacey, not David, and the antifreeze container found on the floor had no prints on it at all.
"If David Castor poured a glass of antifreeze, then why isn't there one fingerprint on that container?" Spinelli said.
And that seemingly new turkey baster found in the garbage can had no prints on it either, but it did have David Castor's DNA on the tip.
Investigators said that Castor's death was no longer a suicide. It was a homicide. District Attorney William Fitzpatrick found Castor's decision to torture himself slowly with antifreeze using a turkey baster one drop at a time unbelievable.
"Suicide by turkey baster is not something I've even heard of in my career," he told ABC News.
As summer turned to fall, the quiet investigation into who could have killed David Castor began as his wife buried her second husband in a small cemetery tucked away in rural upstate New York, right next to her first husband, Michael Wallace. Wallace was 38 when he died, and police began to wonder if his untimely death was a coincidence or a pattern.
Over the next two years, investigators built a case against Stacey Castor, and in September 2007, she was arrested after what they believe was a desperate attempt to throw cops off her trail. She was ultimately convicted of second degree murder in David Castor's death and was also convicted of the attempted murder of her daughter Ashley.
"20/20" was granted full access to all sides of this curious investigation and trial. Stacey Castor talked about the trial and her conviction for the first time 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.