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Caleb Moore, who died Thursday at 25, was no stranger to the dangers inherent in the sports he loved and the level of risk he accepted by participating in them. The young rider from Krum, Texas, grew up racing ATV four-wheelers and practicing freestyle tricks on the quads with his younger brother, Colten.

He landed his first backflip on an ATV in 2006, sparking an around-the-world adventure as the Moore brothers set out with their ATVs to perform on the Crusty Demons and Nuclear Cowboyz tours, among others. By 2012 Caleb was racking up 60,000 frequent-flier miles a year traveling to perform in freestyle shows in locations ranging from Mexico and Costa Rica to Sweden and Russia. Along the way he amassed a list of career injuries to match: a broken back, a torn ACL in his left knee, a broken ankle and broken wrist, and a total of eight concussions, according to ESPN Research. But Moore was undeterred, especially after discovering he could translate his freestyle experience on an ATV quad into a string of podium finishes in the X Games' Snowmobile Freestyle event.

"It was a whole new world. And it was cold," Caleb recalled in a 2012 interview with ESPN The Magazine reporter Alyssa Roenigk, after winning his second bronze in 2011. A year later, he won a third bronze in the Freestyle event and picked up a silver medal in the Snowmobile Best Trick competition. By then the Moore brothers' story had already achieved mythic status among X Games fans: After seeing Snowmobile Freestyle on TV, the duo worked with their sponsor, Polaris (a manufacturer of ATV quads and snowmobiles), to make the transition with new sleds and had a last-minute training session at Evolution Sled Park in Michigan. Two guys from Texas coming out of nowhere to compete on snowmobiles made for an unusual story, but Caleb rewrote the ending, winning bronze in his X Games Aspen debut in 2010, just 32 days after his first ride on a snowmobile.

His early successes in the new sport only left him hungrier: After winning three bronze medals and one silver, Moore wanted more. "I wake up every morning, I work out, I try to eat healthier, I'm completely out of party mode. I don't go out," he told X Games Research before this year's event. "I'm strictly focused on trying to reach that holy grail of a gold medal."

Moore's death Thursday, following head and heart injuries sustained during the Snowmobile Freestyle finals Jan. 24 at X Games Aspen 2013, has reopened the debate about the extreme limits of action sports. Moore will be remembered as the first person to land backflips on ATV quads, motorcycles and snowmobiles, but he'll also be remembered as the first fatality in the 18-year history of the X Games.

X Games officials have said they'll be conducting a thorough review of the Snowmobile Freestyle discipline and adopting any appropriate changes to future X Games. Action sports have always been about pushing the limits; Moore pushed them as far as anybody.

"Caleb and his brother were the real deal right from the start: great guys and terrific athletes with amazing showmanship," Australian FMX star Jackson "Jacko" Strong told ESPN.com on Wednesday. Strong and the Moore brothers were Rockstar Energy teammates who first met while performing together on the Crusty Demons tour in Australia. In December Caleb and Colten mentored the X Games Moto X Best Trick two-time gold medalist as he made his own attempt at crossing over from dirt to snow. On Wednesday, before Caleb's death, Strong put the sled he rode at X Games Aspen 2013 up for auction on eBay to help raise funds for Moore's family to pay his medical expenses.

"Everyone knows what could happen before they start riding, and unfortunately this time someone got hurt badly," Strong said before Moore died.

Moore accepted and even embraced the risks of his sport. In 2012 he competed at Winter X Games with a cracked pelvis he'd injured during practice for a freestyle show and a broken tailbone he injured while practicing in Aspen. "When I look at highlights from last year, everything I was doing was completely crooked," he told X Games Research before this year's event. "I was landing crooked, I was getting thrown off and pretty much my whole body was out of whack. I didn't want anyone to know about it. I didn't want anyone to keep me out of the competition."

A year later and determined to win that elusive gold medal, Moore came up short on a backflip attempt in the Snowmobile Freestyle finals on opening night of X Games Aspen 2013, smashing his sled headlong into the snow. X Games fans gasped collectively as his sled tumbled over him and knocked him unconscious briefly. They then breathed a sigh of relief when Moore walked away from the crash with assistance. But his injuries were deeper than anybody realized at the time: Moore was later treated for a concussion at Aspen Valley Hospital, then flown to St. Mary's Hospital in Grand Junction, Colo., for surgery related to a heart contusion and, later in the week, suffered undisclosed brain injury complications. He died Thursday morning, according to a statement issued on behalf of the family.

Caleb Moore is survived by his parents, Wade and Michele, and brother, Colten, 23.

Stunned X Games athletes and other celebrities -- ranging from Tim Tebow to Lil Wayne -- flooded Twitter on Thursday with messages in response to the news. "My deepest condolences to the family of Caleb Moore," wrote former X Games host Sal Masekela. "He was a wonderful kid that loved what he did with all his heart."