-- Doug Nash held his wife Sylvia as her eyes rolled back and she slumped in his arms. Just hours before she had been healthy and happy, he said, but he knew then that his partner of five years was dead.
"In my arms. That'll haunt me for the rest of my life," he told ABC News in an interview to be broadcast Friday on "20/20." "The vision of her face, just inches away from mine, and those eyes suddenly de-focusing on mine."
Doug said he believes that his wife was poisoned by what a fringe church calls a "miracle cure" for a number of serious diseases -- a chemical solution she took just minutes before suffering the first symptoms that ended in her death in 2009.
TUNE IN to ABC News' "20/20" Friday at 10 p.m. ET for the full report from the Brian Ross Investigative Unit.
The church, the Genesis II Church of Health and Healing, denies Sylvia's death was caused by their holy elixir, known as Miracle Mineral Solution or MMS, and maintains it can be used "to overcome most diseases known to mankind," including cancer, Parkinson's disease, and autism in children.
The U.S. government and medical experts say MMS is really nothing more than a kind of industrial bleach -- a mixture of sodium chlorite and water -- and, when used as directed can cause serious harm to a person's health.
"They might as well be selling Clorox," said Ben Mizer of the U.S. Department of Justice. "You wouldn't drink Clorox, so there is no reason you should drink MMS."
While Sylvia's autopsy was inconclusive, Doug said there is "no question in [his] mind" about what happened that day seven years ago.
"MMS did kill my wife," he said.
Death in Paradise
Doug and Sylvia Nash had planned to sail around the world, beginning in late 2004 and taking their time heading south from California and then west. Sylvia had been a crewmate at the start, but Doug said that just six months into the trip, they were "connected." "And eventually, we married," he said.
After loitering off South America and a two-year spell in New Zealand, they made their fateful stop in the Vanuatu islands in August 2009.
Doug said Sylvia wanted to take measures against malaria, but hadn't liked the medicine she had taken once before. Instead, she met fellow travelers on one of the islands selling something else.
"She thought it was a valid medicine. Turned out, it wasn't," Nash said.
Doug said one day Sylvia decided to drink the MMS mixture after reading the directions on the bottle and immediately hated the taste. "She said, 'Oh my God, that's awful,'" Doug said.
"About 15 minutes later, she began to indicate that she was feeling bad. And that began a sequence of her getting worse and worse throughout the day," Doug said.
Doug said Sylvia suffered diarrhea, nausea and vomiting -- but those were all symptoms that proponents of MMS say are not uncommon and can be an indication that the treatment is working. Doug said Sylvia still thought everything was fine.
"But it didn't get better," Doug said. "It got worse and worse. And by mid-afternoon, she was in... lots of pain. And by evening she was starting to be serious. And by this time I realized she was suffering from poison."
Doug said he called for help on his radio, but by then Sylvia appeared to be losing consciousness. That's when he thinks she died in his arms.
Still, some people came to help and attempted CPR. Eventually a local nurse arrived and gave her a shot of what Doug believed to be adrenaline. But nothing happened. Doug said that the nurse told him, "There's no point. She's gone."
Just the night before, Doug said, 56-year-old Sylvia had been on one of the islands dancing with the local kids, vibrant and full of life.
"There's no doubt in my mind that this stuff [MMS] is what caused her death," he said.
Tracing the Chemicals Back to the US
Doug got Sylvia's body back to a local hospital and an autopsy was performed by a specialist from Australia.
Though a "concentrated solution" of sodium chlorite -- the main ingredient to MMS -- was found in her system, the doctor said Sylvia's cause of death was "unascertained."
But Doug was convinced the MMS was to blame and went on the hunt, tracking the bottles Sylvia had purchased to a corporation called Project Greenlife in Carson City, Nevada.
In 2011, Doug filed a report with the U.S. Food and Drug Administration (FDA). But federal authorities were already on the scent, Doug said, and in 2013 prosecutors indicted Project Greenlife manager Louis Daniel Smith on a number of counts related to a conspiracy to get rich by "marketing [a] toxic chemical as a miracle cure," according to the Department of Justice. Two years later Smith was convicted on four counts and sentenced to more than four years in prison.
"The verdict demonstrates that the Department of Justice will prosecute those who sell dangerous chemicals as miracle cures to sick people and their desperate loved ones," said Mizer, who was involved in the case at the time.
Smith was not charged with Sylvia's death but during court proceedings, the government alleged, "Mr. Smith’s actions have posed a risk of danger to others, including contributing to the death of at least one individual," presumably referring to Sylvia.
Experts: MMS Is a Danger to Adults, Children
Smith may have been thrown behind bars, but the MMS movement is still very much alive and active.
Self-described "archbishops" at the MMS-centric Genesis II Church of Health and Healing, which was founded in 2009 or 2010, claim that thousands of people with debilitating conditions have been treated or cured by the chemical solution.
Doug told ABC News that after he learned more about MMS following his wife's death, he predicted a church would spring up around it.
"It's a money-making operation," Doug said.
Unlike Project Greenlife, which directly sold MMS, the church only asks for "donations" in return for their MMS sacrament. The church has produced slick online videos featuring testimonial after testimonial from people who say the chemicals have helped them. One website offered five sets of chemicals to make MMS for "donations" of $95.99.
One of the church's leading figures, archbishop Mark Grenon, said in an online video that he believed all churches were beyond law enforcement's reach.
"The church is under no law. That's why you can go to a church and get political asylum. A priest can give a kid alcohol, a minor, in public, and not get arrested," he said.
When ABC News attempted to ask Grenon about his church, he denied that the church was doing anything wrong, stood by the MMS "miracle cure" claims and accused an ABC News reporter of being an "actor" and "pawn" of the pharmaceutical industry.
Still on the MMS trail, Department of Justice's Mizer said in an interview with ABC News that wrapping potentially criminal activity in the cloak of religion won't save anyone.
"They can still be prosecuted, yes, if they are selling it in order to cure diseases and are telling people that it will cure diseases," he said.
In the meantime, Doug warned others not to fall for the church's claims.
"Don't do it. Because it may harm you," he said. "Because it harmed me greatly by causing my lover and my life and my partner in sailing to be no longer with me."