The crash happened at about 4:40 p.m. on the state Highway 4 Bypass near Lone Tree Way.
A silver 1995 Honda Civic was driving west on the bypass alongside a black Ford F-250 pickup truck that was towing a boat, possibly a black-and-white boat used for wakeboarding, Maguire said.
The driver of the pickup swerved into the lane occupied by the Honda, and the driver of the Honda, a 19-year-old Antioch woman, veered away to avoid a crash.
The Honda crossed over the median and entered the eastbound lanes of the bypass, where it struck a motorcycle head-on, Maguire said. The driver of the motorcycle and a passenger were both thrown from the bike.
The motorcycle driver was pronounced dead at the scene. His female passenger was taken to Kaiser Permanente Antioch Medical Center, where she died a short time later.
The driver of the Honda was also taken to Kaiser with injuries to her legs. She is expected to survive, Maguire said.
As of this morning, the Contra Costa County coroner's office was still trying to locate the family members of the two people who were killed before releasing their names to the public, a deputy coroner said.
Although the CHP is still investigating the crash and has not determined who was at fault, investigators want to talk to the driver of the pickup truck, Maguire said.
Portions of the bypass through Antioch and Brentwood and numerous on-ramps and off-ramps connected to it were shut down until about 12:45 a.m. while the investigation was carried out.