The chain-reaction accident happened near Towne Avenue just after 8 p.m. At least one vehicle was fully engulfed in flames.
The California Highway Patrol said there were three cars that were initially involved in an accident. That accident was then followed by the crash that involved the big rig and four other vehicles.
"Cars were being hit left and right, and then we saw a truck coming at us. My kids were in the back of the car. The truck hit us and it hit another van. We saw the truck catch on fire. My wife just goes, 'Well get out of here, just keep driving and pull over,'" said driver Richard Arena.
Four drivers were critically injured. Two of them were airlifted to area hospitals. Five other people suffered moderate injuries.
"Some of the witness accounts were that a number of motorists that were driving by heroically got out of their own vehicles and assisted the other drivers out of their vehicles," said Capt. Roland Sprewell with the Los Angeles County Fire Department.
Drivers reported hearing explosions. Firefighters said the sound was that of tires popping. The truck was loaded with 60 gallons of diesel and was hauling wood product.
Firefighters hosed down the contents of the truck to make sure the fire was completely out. The cause of crash remains under investigation.
The crash initially shut down all lanes of the freeway, but westbound lanes were quickly re-opened. Eastbound lanes were still closed as of 11 p.m. The closure begins at the Indian Hill exit. Officials said they hope to have the scene cleared before the morning commute.