Officers responded to reports of a crash involving as many as four vehicles on northbound Highway 280 near state Highway 1 at about 8:15 a.m., CHP Officer Scott Cakebread said.
One person has been confirmed killed, he said. There were no immediate reports of other injuries.
Three left lanes were blocked as emergency crews responded, he said.
A Sig-alert was issued at 8:27 a.m. and was expected to be in effect for about two hours.
