The accident happened at 3:41 a.m. on the northbound 101 Freeway at Oak Street. Authorities say three patients, including the person with the life-threatening injury, were taken to Ventura County Medical Center. The three remaining victims were transported to Community Memorial Hospital.
A fuel tank on the semi ruptured in the accident, spilling about 40 gallons of fuel. Crews were able to stop the fuel from entering a nearby storm drain. CalTrans completed the cleanup and assessed road conditions. Two lanes were closed at Oak Street.
The California Highway Patrol is investigating what caused the accident.