Chelsea Dunn was one of the bright lights in her family -- athletic, opinionated and talented. The 13-year-old was the classic all-American kid growing up just outside Boise, Idaho. A twin to her brother, Hunter, and a big sister to 5-year-old Balee, Chelsea was already mapping out her future. Her parents, Joe and Tammy Dunn, were proud of their vibrant young daughter, who had been accepted to a new charter school earlier this year, and dreamed of becoming an artist.
On the night of April 14 the family's ideal life took a tragic turn. Chelsea headed off to bed while her family was watching television and her mom gave her a kiss goodnight.
When Chelsea didn't come out of her room for breakfast the next morning, her father asked Hunter to get her out of bed.
Hunter discovered a ghastly scene in his twin sister's room. Chelsea was hanging from her closet door with a belt around her neck. She appeared to be dead.
Hunter, stunned, told his father to look in the closet. "I heard Joe screaming. I went to the entryway to the door, and I looked over and saw her. And I turned away really fast and ran to the phone and dialed 911," Tammy told "20/20's" Deborah Roberts.
Police and paramedics rushed to the house only to discover that Chelsea had died the night before, minutes after saying her last goodnight.
The devastated family was haunted by questions. How could their sweet, lively girl take her own life and why? As the family searched Chelsea's room for clues they made a startling discovery.
They found a note Chelsea had written to a friend. She wrote, "I love doing that pass out thing. You wake up and you forget what happened. It comes back though you're all tingly."
"I said that's it. This is exactly what we're looking for. This explains everything," Laura Cooper, a close family friend told "20/20."
Joe and Tammy were shocked by the note. "My husband had gone in to talk to her brother and he had confirmed that she had confided in him that she was playing the game with friends in the PE locker room," Tammy said.
But what was this mysterious "pass-out" game? Tammy had never heard of it, but Joe had. "I saw kids do that when I was in the sixth grade at the elementary school I went to. I didn't have the slightest idea that it would still be going on today," he said.
Apparently, it's a popular game with kids around the world and it goes by a lot of names. "They can be known as space monkey, space cowboy, knockout, gasp, rising sun. In Ireland, it's known as the American dream game," said Dr. Thomas Andrew, an expert on the dangers of this high-risk game.
Andrew says children play it by squeezing a friend's chest or neck to cut off the flow of oxygen. "While the brain is deprived of oxygen, you'll get this sensation of light-headedness. Perhaps numbness and tingling. And if all goes as planned, the pressure's then released. Blood goes torrenting up those carotid arteries and it goes into the brain, and you have this big rush," he said.
That rush is what children seem to crave. Some want it so much they're now often playing the game alone, using shoelaces, ropes, dog leashes, bed sheets and belts. Andrew says the result can be fatal -- and easily confused with suicide.
"In examining the bodies of these children, there is nothing at the autopsy table that will distinguish an asphyxial hanging, due to an intentionally self-destructive act, suicide, from an unintentional catastrophe from playing this game," he said.
In fact, four years ago, Andrew, New Hampshire's chief medical examiner, came across this situation in his investigation into the shocking death of 11-year-old Thomas Fortin. At first glance, the death of the healthy, active boy appeared to be suicide.
The boy's mom, Penny Fortin, remembers a scene eerily similar to the site in Chelsea Dunn's bedroom.
"The first thing I saw was it looked like Thomas was kneeling up against the wall, facing the wall. I remember running over to him and taking him down, removing the dog leash from the coat rack, removing the other part from his neck," she said.
The sixth-grader's death was yet another in a string of similar apparent childhood suicides. Suspicious, Andrew investigated further and soon discovered that some of the kids had been playing the suffocation game to get a quick high.
Andrew demonstrated how children playing the game alone have only moments to undo the choke-hold around their neck before passing out.
If the child fails to unloosen himself, the result can be deadly. "Should he not be able to reach it, and lose consciousness and fall forward, that pressure on his neck is now even tighter. It will deprive the blood flow, or it will deprive the brain of blood flow nearly completely, leading to a fatal result," he said.
That's what happened to Thomas, and most likely to Chelsea, and other pre-teen and early adolescent children around the country. Recently, a 13-year-old California boy and a 10-year-old boy in eastern Idaho were likely victims of the game.
And 14-year-old Jennifer Cernekee from Wisconsin also died under similar circumstances in 2001.
There are tip-offs parents and friends may be able to spot. Experts say severe headaches, marks on the neck, bloodshot eyes and closed doors are common signs. Most importantly, experts say, parents should talk to their children about risky behavior.
Investigators have ruled the cause of Chelsea's death undetermined, but her parents believe there's no uncertainty. They're convinced they lost their daughter to this dangerous game.
"She might have thought it was dangerous, and she might have thought it was risky. But I don't think she had any idea that it ever killed anybody," her father said.
Three months after Chelsea's death, her room is just as she left it. Her drawings still decorate the wall and her birthday money sits neatly on her dresser. Chelsea would have turned 14 the day after her funeral.
And her family is still dealing with the intense grief of losing their vibrant young girl.
Tammy Dunn wishes she would have spotted some warning signs that Chelsea was risking her life for a senseless game. "I blame myself. Maybe I should have known, maybe I should have checked in on her, but I'm angry with her. I'm angry that she made those choices that she made that took her away from us. And her brother and her sister, and everybody who loves her. She had a huge future that you know, with one decision took all that away."