Chewelah Police said multiple agencies responded to the site, southeast of Highway 395 on Huffman Road, where the vehicle rolled over at the top of a hill.
Police had not released the name of the victim on Sunday.
The MountainMania Mud Bogs was advertised as a three-day event, complete with live music, competitions and at least $2,000 in prizes. Witnesses said a driver flipped his vehicle while trying to do a wheelie on a hill climb. The driver was not wearing a seatbelt according to witnesses.
Law enforcement officials said four people were in a jeep when it climbed up a hill and rolled backwards, ejecting one passenger. They said the passenger was crushed and the driver was airlifted to a Spokane hospital. Officials said the other two passengers were a man and a young boy. Officials said those two passengers did not suffer any injuries.
The event organizers said some of the prize money was donated to the victim's family on Sunday. The owners of Mountain Mania Mudsports assured KREM 2 News that they had a strict seat belt policy.
Stevens County Sheriff s Office was the lead agency investigating the incident.