This story previously aired on March 5, 2011. It was updated on Jan. 21, 2012.
Produced by Jay Young, Patti Aronofsky and Sarah Prior
It was the Caribbean vacation Shelley Tyre had been looking forward to - sunning, sailing and scuba diving with her husband and friends in Tortola. And in March 1999, it finally happened.
"They put together a really fun trip, got to do a lot of different things, where they were sailing all around the Caribbean, around the British Virgin Islands. And they were diving off the sailboat they were living on," said Jen Bloom.
But on that fateful last dive, at a site called Twin Tugs, something went horribly, mysteriously wrong. Shelley was dead and the Tortola police wanted to talk to her husband, David Swain.
He called his daughter, Jen.
"He said, 'I'm so sorry. I went with her. I don't know what happened. But Shelley died today.' And he was crying and he was very upset," Jen recalled. "He was doing his best to get her home. He said, 'the autopsy's gonna happen. And there's cops.'"
Tortola police began questioning David about every detail of the dive, and the local medical examiner performed the autopsy. But no one could figure out just how Shelley died.
After a few days, with no evidence of foul play, authorities ruled Shelley's death an accident and released her body to her husband.
"He had a really rough flight," Jen said. "He had to load the coffin onto the plane himself."
David's son, Jeremy Swain, recalls what happened when his father got off the plane. "He came over to me and Jen and he gave us a hug. And he said, 'It's all right. It's gonna be all right.' And he was genuinely hurt."
Shelley Tyre was small in stature, but her impact on David and his children was enormous. And 10 years after her death, David still remembers why he fell in love.
"She had a heart bigger than her. She had passions and drive," David said, pausing, "that had no equal."
A middle school principal at prestigious Thayer Academy outside Boston, Shelley was anything but straight laced.
Every New Year's Day, on a frigid Rhode Island beach, Shelley stripped down to her swimsuit for the Penguin Plunge.
"Five-hundred to 1,000 people all go racing in to the 37-degree water," Jeremy explained. "The water hits you - you get that 'ugh' sort shock right? Shelley turned this beet red color, goose bumps head-to-toe, shaking uncontrollably with this ear-to-ear grin."
With such a love of adventure, it was no wonder Shelley found David Swain attractive. When she met him in the early 1990s, the divorced father of two teens was running a dive shop in Jamestown, R.I. The ocean was his passion.
"My father is a teacher - he's a naturalist by nature," Jen said. "He taught me and my brother that the most important thing in the world is to teach and share what you know and be excited about the ocean."
On one of his kayaking tours, Shelley went along.
"Suddenly there's this lightning storm going on," Jen explained. "And my father just took charge of the whole thing and attached his boat to her boat and rowed them out of it."
Shelley was impressed by David's courage and leadership. He moved in, and in October 1993, they were married. Back then, Shelley was earning far more than David, who says he sank nearly every dime into the dive shop.
Shelley's parents were worried. So David signed a pre-nuptial agreement; he would get no money if he and Shelley divorced.
According to Jeremy, his dad "never cared about money.
"Shelley, herself, couldn't have cared less about money," Jen added. "She just decided this is it. This is the guy I want to be with. He loves adventures. He's not afraid and he'll always be there."
But on that dive in Tortola, even David admits he wasn't there when Shelley needed him. While their friends stayed on the boat, David and Shelley dove into the water.
"We swam our course that we had all agreed upon. Got to the wreck, and we went our separate ways, as we always did," David told "48 Hours Mystery" correspondent Troy Roberts.
David says he has no idea how Shelley died, because he wasn't with her. That's exactly what he told her parents, Richard and Lisa Tyre.
"We got a telephone call, and it was David," said Lisa Tyre.
Adds Richard Tyre, "He said, 'Shelley's no longer with us,' and he indicated he wasn't there and didn't know what happened."
"I knew they were unhappy with me when I made the phone call that night," David said of the Tyres. And he knew he'd have to see them in person to explain.
"He said, 'I don't think they're gonna take this well.' I said, 'Of course they're not. They lost their daughter,'" said Jen.
So one day after bringing his wife's body home from Tortola, David went to see the Tyres.
"We just assumed that divers always had buddies. The buddy system," said Richard. "I kept saying, 'Well, weren't you the buddy?' And he said, 'No, I left her.'"
"You said you split up during the dive, as you normally do. Doesn't that go against standard practice?" Roberts asked David.
"How many times have you broken the speed limit?" he replied. "…every diver at one time or another is diving alone."
Shelley had done solo dives before. David says he tried to help her parents understand what could have happened - explaining that even something small like a headache could have caused Shelley to lose control underwater and make a mistake.
When asked if he thinks it's possible that Shelley just panicked, David says, "Sure. Panic is the end result of something else… I don't know what factor got Shelley, but I think some factor got her started, who knows what it was."
In fact, Shelley had panicked on previous dives - she even wrote about it in her dive log, which David gave to her parents. But it didn't seem to help them.
"Because they're not divers," he explained. "Unless you're a diver, this is gonna be so foreign. It would be very foreign."
What was really foreign to her parents was how unemotional and matter of fact David seemed about Shelley's death.
"I'm just not an outwardly warm, fuzzy guy," he said.
"But your wife just died. Your behavior didn't seem appropriate for a man in mourning," Roberts pointed out.
"My daughter saw it, my son saw it. People who know me know that I'm a private person."
"Even when you're in pain?"
"Probably more so when I'm in pain."
But around Jamestown, and at Shelley's school, the talk was about how well David was doing without Shelley.
"This isn't right. You know, he's too elated almost about this. This guy's too happy," said Don Badger, who taught at Thayer Academy.
Badger says David didn't appear to be in mourning, even at the school's memorial service just weeks after Shelley's death. "It wasn't any solemnity to what he was saying and it bothered so many of this there. It really did."
Then, when David collected more than $600,000 from her estate, he started spending a lot of money very visibly - making pricey renovations to the dive shop, taking vacations and even dating.
David's friend and fellow scuba instructor, John Langella, says despite appearances, there's no doubt in his mind that David was grieving.
"People were looking for answers," Langella said. "I knew he was upset. He was not right. He didn't know if he could ever go back in the water."
Sandy Wheeler, David's ex-wife and Jen and Jeremy's mother, has known David for 40 years.
"He called me and said, 'I'm not doing well. When is this pain going go away?'" she explained. "He was in a lot of pain and he didn't know how to handle the pain."
But if he was in pain, the people who needed him to express it most of all were Shelley's parents. And for whatever reason, David just couldn't do it.
"In the year or two after Shelley's death, he was not able to say the things that the Tyres needed to hear," said Jeremy.
David's failure to do so didn't just make Shelley's parents suspicious… it made them angry.
"When I heard through the grapevine that they were starting to throw the word murderer around, I couldn't believe it. I couldn't believe it. I still can't believe it," Jen said. "Every cell in my body wanted to go over there and just like knock on the door and say, 'Are you insane?'"