The cause of a Highway 20 head-on crash that seriously injured four Mendocino County residents remained unclear Tuesday.

"It's still under investigation," CHP spokesman Steve Krul said.

The victims all suffered major injuries in Monday's crash, the CHP said.

The driver of the sedan, Cheryl Johnson, 51, of Ukiah, was flown to UC Davis Medical Center for treatment while her passenger, Linda K. Aseltine, 66, of Ukiah, was taken to Ukiah Valley Medical Center. Caltrans truck driver John D. Frank, 39, of Ukiah and his passenger, Russell Maidrand, 33, of Willits, also were taken to Ukiah Valley Medical Center.

The crash occurred at 8:25 a.m. near Lake Mendocino. The Caltrans truck was westbound and preparing to stop to retrieve a dead deer from the road and the sedan was eastbound when they collided, according to the CHP.

Investigators have not determined which vehicle crossed into the oncoming lane, Krul said.

A man in third vehicle, David Warden, 56, of Potter Valley, drove over some debris but was not injured.

The crash briefly shut down the highway and reduced traffic to one lane for several hours.