Murder or accident? Shelley Tyre's fatal dive

A paradise vacation ends in death and then a conviction, but will a legal twist set the killer free?

Ten years after his fatal vacation with his wife, Shelley, David Swain again has a Caribbean view. But this time it's from inside a cell in Her Majesty's Prison on Tortola. He has been here, awaiting trial, for nearly two years.

"It's a journey that I didn't wanna take, but if I'm forced to take it, I'm going to do the best I can with it," David told Troy Roberts.

His children, Jen Bloom and Jeremy Swain, have come to Tortola to help him prepare his defense.

"He's very hopeful. He believes - he has to believe. All of us have to put 100 percent of our belief into that this will be OK," Jen said. "This is a man's life we're talking about."

Even as Jen and Jeremy are fighting for their father, their stepmother, Shelley Tyre, is still very much on their minds.

"I wanted to go to a place that would be special to me, where I could pay my respects in peace doing something that Shelley and I shared. Because Shelley and I used to dive together, too," Jeremy explained. "We went down the line and I had her wreath on my arm… there was a pipe and I put the wreath down on top of that, made sure it would sit nice… said a few things in my head, and that was it. I said goodbye."

Jeremy said goodbye, but could not let go of a troublesome detail.

"I went back a couple more times because what was coming out really bothered me," he said.

He says he was most bothered by claims that his father was with Shelley when she stopped breathing. Prosecution experts claim that the amount of air left in Shelley's tank showed that she was alive for only 8 minutes.

"I went down to that site… I used that amount of air that she consumed. And it took me 15 minutes," said Jeremy. He says the prosecution's 8-minute calculation doesn't add up. He says Shelley just wouldn't breathe air that quickly.

"Not if she was running a marathon under the water would she have ever used up air that fast," he explained.

The only explanation, according to Jeremy, is that Shelley was alive a lot longer than prosecution experts believe. Meaning David would have been long gone and nearly back to the boat at the time of her death.

"The only thing that everybody can agree on is that some external force was applied to her equipment," said defense attorney Neil Tassel.

For his criminal trial, David has the help of two experienced Boston defense attorneys, Neil Tassel and Tim Bradl. They argue that Shelley could have ripped off her own mask in a panic.

"It's right there in the PADI (Professional Association of Diving Instructors) beginner's manual - panicked divers reject their gear. In other words, they rip their masks off," said Bradl.

When asked if they believe Shelley panicked underwater, Tassel replied, "Absolutely. That's what all the evidence appears to be."

Now, with these lawyers helping to prepare for the criminal trial, David's children are feeling hopeful about their chances.

"We know the truth is on our side. So we actually have the upper hand," Jen said. "If you just stay with the truth, it's all gonna be good."

But staying with the truth will mean talking about another possible motive for murder.

About a year before Shelley's death, David got very friendly with Mary Basler, a chiropractor who visited his dive shop.

"Mary Basler came along, a bright, happy woman willing to talk," said David.

Mary Basler might have been willing to talk, but when David kissed her one summer night, she told him she was not willing to get involved with a married man.

"I'm as guilty of being curious beyond what I should have as anybody," said David.

Perhaps still curious, David wrote Mary that fall, inviting her to spend the weekend with him in Killington, Vt. The letter said:"I promise to return you in a rejuvenated state… or in need of rest… whichever strikes our fancy."

"You invited her to Killington for the weekend. Wasn't that going to be the beginning of something?" Roberts asked.

"No," replied David.

"It was just going to be a friendly weekend in Killington?"

"You make it sound like a guy can't have a girl friend," David replied.

"Did your wife know you invited her?"

"No."

Mary Basler didn't go to Killington, but soon David wrote Mary again, telling her, "I'm wanting to be with you" but "I can't change this mess I've got anytime soon."

Read more of David's letters to Mary Basler

"You made a reference, a mess that you need to get out of," Robert points out.

"Well, I think I made a mess by letting my curiosity get the best of me," David replied.

"You weren't describing your marriage to Shelley as a mess?"

"No."

David addressed that letter to "Soulmate Mary."

"You thought Mary was your soulmate?" Roberts continued.

"It's a nice word, isn't it? said David.

"Did you envision a future with Mary Basler?"

"At that time, no."

But according to both David and Mary Basler, after that kiss, nothing happened between them.

"It was a stupid, inconsiderate mistake," said David.

That is, until Shelley Tyre died. Two months after the fatal trip to Tortola, David Swain and Mary Basler started a relationship.

"When my father started dating Mary, soon after Shelley's death, I remember thinking, 'He's running. He's never dealt with emotion in a strong way. It's starting to come up,'" Jen recalled. "And so he's running to this woman."

David and Basler dated for about a year.

"I don't think it's about whether he was having an affair or not. It's about whether he killed his wife or not," said David's ex-wife, Sandy Wheeler. "I asked him straight out, 'Did you do this?' And he said, ' No, I did not do this.' And I believe him."

When asked what prompted Wheeler to ask her ex-husband if he killed Shelley Tyre, she replies," I think I just needed to hear it from him. I never thought he did it, but I think I just needed to hear it from him."

Wheeler and David Swain were married for 12 years. And in the time they were together, she says she "never" saw a violent side to David.

But it turns out, David's early life was far from peaceful.

Says Wheeler, "David was a person who kind of wanted to leave things… the past is the past."

And now, with his life on the line, David's defense will delve into secrets he has protected for decades.

"His mother died the first year we were together," Wheeler tells Roberts. "She was killed."