Moore, 25, remained in critical condition Tuesday in Grand Junction, Colo., after having failed to convert a flip off a 70-foot ramp with his 450-pound snowmobile in Aspen on Thursday.
After completing five tricks, including a midair full-body stretch from the handlebars and a flip around his seat, Moore flew over the handlebars headfirst into the snow after the front skis of his snowmobile dug into a landing ramp.
At impact, the vehicle rolled onto Moore and appeared to strike him near the head and chest.
Moore eventually walked off the course and was treated for a concussion, but his condition has deteriorated since with bleeding around the heart prompting surgery, and a brain injury emerged, said his family.
"Caleb is not doing good at all," Moore's grandfather Charles Moore told the Denver Post. "The prognosis is not good … it's almost certain he's not going to make it."
If Moore dies, he'd be the first fatality in the 17-year history of the Winter X Games.
The ESPN brand, which includes an X Games summer version at Staples Center and other Southland venues, routinely tests safety boundaries with motorcycle, skiing and skateboard events.
A network spokeswoman said there was nothing faulty about the Aspen course or with Moore's equipment, adding in a statement: "Our thoughts and prayers continue to be with Caleb and the Moore family. We've 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 is a four-time X Games medalist who fell short on his rotation on a move he has landed several times previously."
Dan Lebowitz, executive director of Northeastern University's Center for Study of Sport in Society, said, "I don't know that you can ever divorce the gladiator element in athletes from sport.
"In many respects, sport's essence is about a competitive challenge. Athletes are bigger, faster, stronger, and performance is more spectacular. Part and parcel with that is the Pandora's box of risk attached to these events. It's called the extreme games, after all."
While sports history is dotted with safety advancements — helmets, fewer rounds in boxing, equipment modifications in NASCAR — the X Games are not regulated by a government body like boxing or mixed martial arts.
"Our view is safety is paramount, always has been," said Keith Kizer, executive director of the Nevada State Athletic Commission.
When three boxers died within a year of each other in 2004 and 2005 in Nevada, the commission formed a health and safety review that added a third ringside doctor and limited overweight fighters to just one hour of extra weight cutting. No fighter has died in the state since.
Moore, from Krum, Texas, graduated from jumping all-terrain vehicles to the snowmobile, winning bronze medals from 2010 to 2012 in the freestyle event.
When he crashed, an X Games commentator said the terror of a pending crash is "a horrible feeling, there's nothing you can do. You commit to the flip, you're committed."
Moore's brother Colten, another X Games competitor, also crashed and separated his pelvis last week; a skier suffered a fractured spine and a snowboarder had a concussion. Another snowmobiler wrecked and his vehicle accelerated toward the crowd, injuring a boy and crashing into a catch fence.
Competitor Tucker Hibbert, who won his sixth straight SnoCross title at Winter X, said the incident reveals the perils of what he and his peers do.
"You're seeing the most extreme side of our sport," Hibbert said. " … Naturally, you'll see some injuries and some pretty big crashes when you're pushing the limits."
Northeastern's Lebowitz said, "In almost every sector of the world, people push envelopes and it is commendable to push athletic performance too. The push-pull of it is, what envelope is good to push, and what qualifies as crazy?
"I'm not sure this qualifies as a tipping point of going too far. The guys in these events are always looking at, what's the next trick, while our fascination is in, how far can they go? Boxers have died, football players have been paralyzed. You respond to that with safety measures. Maybe engaging in this conversation is how we reconcile this."
The X Games, in a way, are an extension of the public's attraction toward daredevils, such as the massive interest Evel Knievel built in the 1970s.
Boxing promoter Bob Arum promoted Knievel's failed jump across Idaho's Snake River in 1974, profiting from the venture so brazenly he brought in the late famed oddsmaker Jimmy the Greek to set a line on two bets.
"Live," Arum said, "or die."
The Associated Press contributed to this report.