Caleb Moore passed away this morning in Colorado, a family spokesperson said.

Moore's family declined to release further details about the death, but stated on Sunday that he had suffered a "brain complication" stemming from the accident ... on top of a severe concussion and bleeding of the heart.

Moore was competing at the snowmobile freestyle final last Thursday when his 450-pound snowmobile flipped over and landed on top of him.

Caleb -- once an avid all-terrain vehicle rider -- was a 4-time X Games medalist who first began competing in 2010, after just one month of practicing on snowmobiles.