Three Fairfield residents and a Tracy man were killed in a head-on crash on a remote section of Highway 12 in San Joaquin County on Monday night.

"Everybody involved was killed, and there were no witnesses so we're still piecing things together," California Highway Patrol Sgt. Kyle Plant said Tuesday.

The Fairfield residents were ages 23, 28 and 31 and were in a silver 2002 Hyundai Elantra. The Tracy man was 42 and driving a white 2012 Toyota Camry.

The identities of the victims have not been released pending notification of next of kin.

About 9:10 p.m., the vehicles collided on Highway 12 on Brannan Island just east of the Sacramento County line, Plant said. He described the area as "out in the middle of nowhere."

The Hyundai was found resting on its roof in a ditch. The Toyota was found straddling the east and westbound lanes of the highway.

Both vehicles had major front-end damage.

"It was a head-on collision -- headlight to headlight," Plant said.

No one was ejected from the vehicles, and all were pronounced dead at the scene by paramedics.

Highway 12 was closed for about 3½ hours for the investigation and removal of the deceased and vehicles.

Officials don't know whether alcohol was a factor in the collision. The cause of the crash remains under investigation.

Anyone with information regarding the accident can contact the CHP at 209-943-8600.

Contact Jason Sweeney at 925-847-2123. Follow him at Twitter.com/Jason_Sweeney.