Washington (CNN) -- The death of Caleb Moore is shining a spotlight on growing concerns over safety in extreme sports.
The freestyle snowmobiler, attempting a backflip last week at the Winter X Games in Aspen, Colorado, couldn't rotate his machine enough to land the maneuver. The skis dug into the lip of the slope, bringing the 450-pound snowmobile crashing down on him. The vehicle slammed into Moore's head and chest.
After initially diagnosing Moore, 25, with a concussion after Thursday's accident, doctors soon discovered bleeding around his heart.
He had emergency surgery at St. Mary's Hospital in Grand Junction on Friday, a day after the accident. He had brain complications, and his family issued a statement saying he was in critical condition and being closely monitored.
On Thursday, a week after the accident, Moore died.
The combination of quirky, hair-raising events that make up the X Games were once pushed to the distant margins of televised sports. But in recent years, some have been added to both Summer and Winter Olympic slates.
"They do train all year for this," said Dr. Christine Trankiem, a trauma surgeon at Medstar Washington Hospital Center. "But it's important for the folks watching at home to realize that these acts, while exciting to watch, are potentially life-threatening, limb-threatening and brain-threatening acts if an accident should occur."
Moore's injury was one of several at this year's Winter X Games. His younger brother, Colten, suffered a separated pelvis in the same event. Freestyle skier Rose Battersby, a New Zealander, incurred a spinal fracture, and Icelandic snowboarder Halldor Helgason suffered a concussion.
Sebastian Landry, a filmmaker who specialized in snowmobile features, told the New York Times last year that injuries in the sport made him lose interest in filming.
"It seemed like every time we went to the mountains somebody went to the hospital," Landry told the Times. "Kids get stars in their eyes and just want to go for it. That, plus being raised with the motor-head mentality where it's all or nothing."
It's not just the athletes who appear to be at risk.
During one of the jumps on Sunday night, Australian Jack Strong was attempting a backflip on his machine when it got away from him. The runaway snowmobile crashed into a bank of fans. A young boy who hit his knee on a parked vehicle while trying to get away was evaluated and released; other spectators avoided injury. Strong was not seriously injured.
ESPN, which organizes and televises the X Games, says it is constantly working to ensure the safety of competitors.
On Thursday, the network said it was deeply saddened by his death. "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, adding that Moore was "a four-time X Games medalist who fell short on his rotation on a move he has landed several times previously."
ESPN says the safety of fans is also a priority.
"We've worked closely with athletes, risk management specialists, sport organizers and event managers to ensure the safest possible conditions for athletes and spectators alike. Further measures are constantly being evaluated to ensure the safest possible experience for spectators," the network said in an earlier statement.
One thing spectators won't experience: a drop-off of the X Games.
ESPN says it is expanding the Summer X Games to Munich, Germany; Barcelona, Spain; Foz do Iguacu, Brazil; and Los Angeles. The Winter X Games were expanded this year to include Aspen and Tignes, France.
"This is something that is truly valuable for sponsors, for ESPN, in terms of generating tens of millions of dollars of revenue, in terms of subscriber fees and advertising sales," said Lee Berke, president and CEO of LHB Sports, Entertainment & Media Inc.
Berke said that while ESPN did not win the U.S. rights to televise the Olympics, "they've gone about making the X Games their own Olympics."
CNN's Josh Levs contributed to this report.