The annual winter competition, this year at Aspen's Buttermilk ski area, has had a somber feel since Thursday's accident in which Moore, 25, crashed his snowmobile in freestyle competition, The Denver Post reported Wednesday.
Sunday it was announced his condition worsened as a cardiac injury led to a brain complication. He remained in critical condition in a Grand Junction, Colo., hospital, the newspaper said.
A New Zealand skier, Rose Battersby, 19, sustained a fractured back while practicing Sunday.
"When is enough enough?" asked Paul Thacker, 36, of Anchorage, Alaska, back at the X Games to compete in adaptive snowmobile racing after a 2010 accident left him a paraplegic. "It's kind of like you tell me I can't do something and I'll show you how I can. It's how we are wired."
Thacker formerly held the world record for jumping his snowmobile 301 feet.
Assessing and mitigating risk is a big part of the X Games, the newspaper noted, as athletes raise the bar with gravity-defying artistry once thought impossible.
"These are inherent risks in the sport, and I wouldn't do it if I didn't believe in myself," said Canadian medal-winner Kaya Turski. "Still, it is a fine line, and I've crossed that line a few times."