A casino bus plowed through a guard rail on a highway east of San Diego and landed in a drainage ditch, seriously injuring several people, authorities said.

The shuttle from Barona Valley Ranch Resort & Casino crashed when the driver swerved on state Route 67 late Monday, the California Highway Patrol said.

Three bus passengers and their driver, identified as Michael Dorsey, 38, of San Diego, suffered serious injuries. Other passengers among the 15 aboard suffered minor to moderate injuries and were taken to local hospitals.

A pickup truck also skidded off the highway and rolled down an embankment when its 17-year-old driver tried to avoid hitting the bus. The teen suffered major head trauma, according to the Highway Patrol.

Authorities were investigating what caused the driver to swerve.

A Barona spokeswoman did not immediately return a message left on her cell phone early Tuesday.