Police said a Ford Mustang was likely racing another car moments before blowing through a stop sign at Latigo and Maulhardt avenues and colliding with an SUV carrying a family of five.
The force of the impact caused the SUV to flip over, trapping the passengers inside.
Firefighters had to use the Jaws of Life to extricate the 35-year-old father, wife, two teenage boys and daughter from the SUV.
"We determined through the investigation that the Mustang failed to stop for the stop sign because it was in a race with another vehicle at the time," Oxnard police Cmdr. Ron Whitney said.
The driver of the Mustang and his 23-year-old passenger were taken to Ventura County Medical Center with broken bones.
The family was transported to the hospital with serious to life-threatening injuries. The girl is in critical but stable condition.
Police are investigating whether drugs or alcohol were a factor.
No one was charged with any crime.