When he died Thursday morning in a Colorado hospital, the news came through simple and sad statements from his family and from ESPN, the network that created the ever-expanding X Games in 1995 as a showcase of action-packed sports.

Moore's death appears to be the first directly related to the X Games, ESPN's high-flying carnivals. It immediately raised cultural questions about the lure of such events to young daredevils, the appeal to viewers and the responsibility of organizers.

A generation ago, such televised feats were the domain of singular performers like Evel Knievel. But with the growth of the X Games and sponsors like Red Bull that have rushed into the death-defying stunt business, the pool of willing participants seems bottomless.

There are now six annual incarnations of the X Games, summer and winter versions broadcast around the world. With snowmobiles and skateboards, snowboards and motorcycles, the events — many of them based on high-flying aerial stunts — seem to invite disaster, with names like SuperPipe, Mega Ramp and Big Air.

The growing mainstream popularity has led some events, like the halfpipe and slopestyle contests in snowboarding and skiing, to be added to the Winter Olympics.

The recent four-day X Games, which ended Sunday, had at least six athletes sent to the hospital in ambulances, including Moore's brother, Colten; a freestyle skier with a broken back; and a snowboarder who flew off the edge of the halfpipe and into the crowd.

A few hours before sustaining the injury that would kill him, Caleb Moore sat in his family's trailer in Aspen, Colo., and described the first time he boarded a machine and performed a back flip. Living in Krum, Tex., he was 19 and on a four-wheel all-terrain vehicle.

"It was the most exciting moment of my life," Moore said. "And the most stressful, heart-pounding moment of my life, too."

The risks are nothing new to so-called extreme athletes in action sports, but the list of severe injuries and deaths only grows. The X Games this year featured tributes to the freestyle skier Sarah Burke of Canada, one of the best in the world, who died last year after sustaining a brain injury in a crash during halfpipe training.

At the 2007 Summer X Games, the introduction of a Mega Ramp for skateboarders — 62 feet tall and 293 feet long — was debated after Jake Brown fell five stories to the wooden contraption and was knocked unconscious. The ramp, and Brown, returned the next year.

In 2009, months after winning a gold medal at the X Games, Jeremy Lusk, a freestyle motocross star, died after crashing on a jump in Costa Rica and sustaining a head injury.

One of the announcers at this year's X Games was Kevin Pearce, whose Olympic hopes and snowboarding career ended three years ago when he sustained a brain injury during practice.

Among all the events, there may be nothing more frightening, or alluring, than snowmobiles — jet-quick and nimble, but also weighing nearly 500 pounds.

"As a result of this accident, we will conduct a thorough review of this discipline and adopt any appropriate changes to future X Games," ESPN said in a statement after Moore's death. "For 18 years, we have worked closely on safety issues with athletes, course designers and other experts. Still, when the world's best compete at the highest level in any sport, risks remain. Caleb was a four-time X Games medalist attempting a move he has landed several times previously."

The freestyle event that Moore participated in featured a series of ramps that launched snowmobilers 100 feet or more into the air. Contestants had 75 seconds to perform gravity-defying tricks to impress judges. Back flips are popular, with variations like letting go of the handlebars or trying to freeze momentarily with the snowmobile straight overhead.

Moore's back flip on one jump fell short. The skis on the front of his snowmobile pierced the top of the ramp, throwing Moore face-first into the snow. The snowmobile somersaulted down the ramp and bounced on top of him.

Moore was knocked unconscious. Surrounded by paramedics, his father and his brother, who was waiting nearby for his turn, Moore regained consciousness and walked gingerly to a medical tent. Feeling woozy, he was placed on a stretcher and put into an ambulance, with a diagnosis of a concussion.

Thirty minutes later, Colten Moore's first run ended in a crash on the same landing ramp. He over-rotated on a back flip, his snowmobile landing hard on its back edge. Moore was hurled off the back.

He was strapped into a gurney and taken to an ambulance, where he joined his brother at Aspen Valley Hospital. Colten received a diagnosis of a separated pelvis and was released the next day.