Moore was performing a flip Thursday when he clipped the top of a jump and went over the handlebars and landed face first into the snow. The snowmobile rolled over him, but he walked off with help and went to a hospital with a concussion.
Moore later developed bleeding around his heart and was flown to a hospital in Grand Junction for surgery. The family later said that Moore also had a complication involving his brain.
"Caleb is not doing good at all." Caleb's grandfather Charles Moore told The Denver Post. "The prognosis is not good at all. It's almost certain he's not going to make it."
A family spokeswoman reissued a statement Tuesday thanking fans, friends and family for their support and asked for continued prayers. The family declined further comment.
A separate accident on Sunday left Moore's younger brother, Colten, with a separated pelvis.
The safety of the snowmobile events at Winter X came has fallen under scrutiny with several recent accidents and mishaps. In addition to the crashes by the Moore brothers, there also was a scary scene when a runaway sled veered into the crowd Sunday night after the rider fell off during a jump gone wrong.
In that incident, snowmobiling newcomer Jackson Strong tumbled off his machine during the best trick competition. The throttle stuck on the 450-pound sled and it swerved straight toward the crowd as fans scurried out of the way.
A young fan was evaluated on site for a right knee injury and released to his father. It's not clear whether the teenager was hurt jumping out of the way or was struck by the sled, which came to a rest when it got tangled up in the retaining fence.
Tucker Hibbert who won his sixth straight SnoCross title at Winter X, hopes all these unfortunate incidents doesn't tarnish the image of snowmobiling.
"Obviously, at the X Games, you're seeing the most extreme side of our sport," said Hibbert, who's from Pelican Rapids, Minn. "It's definitely dangerous and exciting all at the same time. But it's also a lot different than what snowmobiling in general.
"Friends and family riding around, going down the trails, having fun riding snowmobiles, is quite a bit different than hitting a 100-foot ramp and doing double backflips. Naturally, you'll see some injuries and some pretty big crashes when you're pushing the limits."
The spills at Winter X weren't just limited to snowmobiles. Rose Battersby suffered a lumbar spine fracture in a wipeout on a practice run before the skiing slopestyle competition. She was transported to Denver on Sunday and had feeling in all extremities, according to X Games officials.
Soon after her crash, Ashley Battersby, who's not related to Rose, wiped out on the course and slid into the fencing. Battersby was down for at least 30 minutes before being carted off on a sled and taken to a local hospital with a knee injury.
There also was a bad wipeout in the snowboard big air competition, when Halldor Helgason of Iceland suffered a concussion when he over-rotated on a flip. He raised his hand to salute the crowd as he was being taken off the icy course.
Moore's crash came just over a year after one of the most high-profile deaths in the extreme sports community.
Canadian freestyle icon Sarah Burke died Jan. 19, 2012, after sustaining irreversible brain damage in a training accident in Park City, Utah. The 29-year-old was a pioneer in the sport and a driving force behind the inclusion of slopestyle and halfpipe skiing at next year's Winter Games in Sochi.