WALNUT CREEK, Calif. >> A Tesla driver was killed and a passenger was critically injured Saturday when the car plowed into a fire truck that was parked on a Northern California freeway to shield a crew clearing another accident, fire officials said.

Four firefighters who were in the truck when it was struck on Interstate 680 were treated for minor injuries, said Tracie Dutter, assistant chief of the Contra Costa County Fire Protection District.

The driver was declared dead at the scene, Dutter said. The car needed to be cut open to remove the passenger, who was taken to the hospital.

Photos showed the front end of the car was crushed and the $1.4 million ladder truck was damaged.

California Highway Patrol Officer Adam Lane said it was not clear whether the driver may have been intoxicated or whether the Tesla Model S was operating with automation or driving assistance features.

The National Highway Traffic Safety Administration is investigating how Tesla’s Autopilot system detects and responds to emergency vehicles parked on highways. At least 14 Teslas have crashed into emergency vehicles while using the system.

Dutter said the truck had its lights on and was parked diagonally on northbound lanes of the freeway to protect responders to an earlier accident that did not result in injuries.

The fatal accident occurred around 4 a.m., and it took several hours to clear the freeway. The firetruck had to be towed away.

The Model S was among the nearly 363,000 vehicles Tesla recalled on Thursday because of potential flaws in its “Full Self-Driving” system. While the recall is aimed at correcting possible problems at intersections and with speed limits, it comes amid a broader investigation by U.S. safety regulators into Tesla’s automated driving systems.