A woman was killed and her three male passengers seriously injured when her pickup overturned off northbound Highway 101 near the county line early Wednesday morning, tumbling over an embankment to a deep ditch off the road, the CHP said.

Authorities were still trying to confirm the identities of the four people, and no names were being released, the CHP said.

The incident occurred just south of the Sonoma-Marin county line, where a cattle crossing built under the highway creates a substantial drop, Officer Andrew Barclay said. The Toyota truck landed on its wheels but only after it had overturned, he said.

A passenger in the car called for help at 2:15 a.m. but was unable to say where the truck had gone off the highway, launching a 20-minute search by numerous law enforcement and fire agencies along a stretch of the highway between Novato and Petaluma, the CHP said.

The vehicle's location in the ditch made it difficult to see even with spotlights except from the edge of the highway, Barclay said.

Authorities eventually determined from the passenger that the truck had passed the Novato landfill but hadn't reached the gas station at Kastania Road, narrowing the location and leading to discovery of the vehicle.

The driver was pronounced dead at the scene, the CHP said.

A rear passenger also suffered life-threatening injuries and was taken to Marin General Hospital, where he reportedly was in critical but stable condition later in the morning, the CHP said.

Two other passengers were taken to Santa Rosa Memorial Hospital with major injuries, but also were listed in stable condition, authorities said.

Investigation of the crash and removal of the pickup closed the right hand lane of northbound Highway 101 until just after 6 a.m., the CHP said.

All lanes are now open.