Lusk, a Temecula resident, had been in a medically induced coma, with swelling of the brain, at Calderon Guardia Hospital in San Jose. A spokesman at the hospital said he suffered severe brain damage and a possible spinal cord injury.
Nicknamed "Pitbull" because of his tenacity on a motorcycle, Lusk was injured after failing to fully rotate a back-flip variation while soaring over a 100-foot jump.
He slammed headfirst into the dirt on the landing ramp's down-slope. It was reminiscent of a similar crash he endured while attempting the same trick during the 2007 X Games at the Home Depot Center in Carson, but Lusk walked away from that incident.
The trick involves extending the body away from the motorcycle and grabbing the seat as the motorcycle is upside down, then pulling back aboard as the motorcycle is righted before landing. Lusk clearly had trouble getting back on the seat, and some witnesses said swirling winds within San Jose's Ricardo Saprissa Stadium may have been a factor.
Despite the danger associated with freestyle motocross, Lusk is believed to be the first pro rider to have died from injuries suffered in an FMX contest, though several have incurred serious injuries.
Lusk, who was born in San Diego in 1984 and had been riding motorcycles since he was 3, turned pro at 19. He was coming off his most successful year.
He won his first X Games gold medal in the freestyle motocross competition last summer at the Home Depot Center, and won a silver medal in the FMX best-trick contest.
Also in 2008, Lusk won two gold medals in an X Games Mexico competition; he finished third in the Dew Tour series standings, and he was never lower than fourth in a string of high-level international events.
The X-Knights competition Saturday in Costa Rica was not affiliated with the X Games.
Of Lusk, X Games General Manager Chris Stiepock said: "He was really starting to emerge as one of the premier freestyle motocross riders in the world.
"He was a great kid and a great athlete. I think he represented his sport very well. He was very passionate about what he did, and I think it's a great loss for the freestyle motocross community for sure."
Stiepock said he did not believe the increasingly risky maneuvers attempted by freestyle motocross riders have pushed the sport over the line. "I think there's a whole lot more that can be done with freestyle motocross, and we're going to continue to feature it strongly at the X Games," he said.
Lusk was on a team of star FMX riders called Metal Mulisha, founded by the sport's most iconic figure, Brian Deegan.
Deegan was in Costa Rica and unavailable for comment but released a statement through his publicist: "Jeremy motivated me to be a better person; he was my best friend. The bond of this team is one that will never be broken."
Lusk is survived by his wife, Lauren, and his parents, who were with him when he died.
Memorial services are pending. The Jeremy Lusk Trust Fund has been set up within the Athlete Recovery Fund. Donations can be made via the website athleterecoveryfund.com.