The bus driver told the CHP his vehicle suffered brake failure and he was unable to slow on a curve and instead smashed into a guard rail, then trees and boulders along the road.
The bus was headed to the Casino Pauma with 18 passengers.
The passengers were all from the Los Angeles area, and the driver lives in El Monte, according to the Los Angeles Times.
The injured were taken to Palomar Medical Center in Escondido, according to authorities.
Neither drugs nor alcohol appeared to be involved, authorities said.