A woman was killed Sunday afternoon when her car went off the Almaden Expressway onramp to state Highway 85 and landed in an irrigation pond, according to a California Highway Patrol spokesman.

Witnesses say the vehicle, which appears to have been traveling from a nearby Costco store, failed to make a turn on the ramp and instead drove straight up a curb, through a fence and into the pond around 2:40 p.m., CHP Officer DJ Sarabia said Sunday.

The car became submerged in the pond, which Sarabia said serves as a holding tank for the Santa Clara Valley Water District.

Bystanders and emergency responders tried to reach the driver, who was trapped inside, but were unsuccessful.

Sarabia said the 69-year-old woman, whose identity has not yet been released, was wearing her seat belt and was removed from the vehicle by divers around 3:15 p.m. and taken to Santa Clara Valley Medical Center, where she was declared dead.

Although initial reports suggested there might be more than one person in the car, the CHP said the woman was a solo drover.

The CHP is currently investigating whether the crash was due to a mechanical problem or medical emergency.