Authorities say the bus blew a tire and rolled over near state Route 371 on Monday night, about 10 miles east of Temecula.
Riverside County Fire Department spokeswoman Jody Hagemann says four people were hospitalized, two with critical injuries and two moderately injured. Twelve people with minor injuries were treated at the scene.
The bus was carrying a crew from the Oak Glen Fire Camp in Yucaipa.
Earlier Monday, the California Department of Forestry and Fire Protection had been fighting an 20-acre wildfire near the crash site. It was unclear Tuesday morning whether the crew on the bus had been at that fire.