A driver was fatally shot Saturday night on Highway 4 in Antioch, California Highway Patrol officials said.

The victim, who was not identified, was behind the wheel of a Toyota Solara when an assailant opened fire shortly after 9 p.m. just west of Hillcrest Avenue, the CHP said.

Officers and paramedics found the victim dead in the Toyota at the Hillcrest Avenue off-ramp.

No suspect was identified or arrested in the slaying.

Anyone with information about the killing is asked to call CHP investigators at (800) 835-5247.

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com