Authorities responded to the area of Pleasant Valley Road and Las Posas Road at about 6 a.m. to an initial report of five parties trapped in the collision. Officials said it was dark and foggy at the time of the crash.
According to preliminary investigation, a Camry was headed eastbound on Pleasant Valley Road and was attempting to pass a car. Because a Cadillac was approaching, it went back into its lane, but then the driver decided to make a second attempt.
"It tried again, came back out, and must have underestimated how close the Cadillac was, panicked, swerved, lost control of the vehicle, ending up in the westbound lane being broadsided by the Cadillac," said Ventura County Sheriff's Senior Deputy Julie Novak.
The posted speed limit is 55 miles per hour. The estimated speed of the vehicles involved was between 55 and 60 mph, according to investigators.
The dark and foggy early-morning conditions on the rural road were described as "unsafe" by investigators.
Five people were in the Camry. Three people in their early 20s were declared dead at the scene, while two others in the Camry were taken to a local hospital in critical condition.
One of the hospitalized men was declared dead later Monday. The second man remained in critical condition.
Investigators say the five men in the Camry were on their way to work.
The driver of the Cadillac, a man in his 70s, was hospitalized with minor injuries.
The Ventura County Coroner's Office did not release the names of the deceased men pending notification of families.