A tractor-trailer carrying produce jackknifed and clipped several cars. The rig burst into flames, and its trailer separated and flipped onto the southbound lanes of the 5, shutting down all lanes of the freeway.
Occupants of two cars were trapped for several minutes until rescuers arrived.
One trucker stopped to try to help the victims.
"I heard a bang, and I looked at my left side mirror, and what I saw was a trucker was heading right in the middle of the divider. I just pulled over 'cause I saw the truck crash, and I saw flames coming out from the bottom of the truck," said trucker Jose Viccarra.
The truck driver escaped with minor injuries, but firefighters needed 20 minutes to free at least two passengers of another vehicle. Seven people were injured, two in critical condition.
Caltrans worked to clean up the freeway for several hours and was able to reopen the 5 before Friday's morning commute began.