According to the California Highway Patrol, the crash near Westminster Boulevard involved at least four vehicles and was reported about 2 a.m. A pickup truck overturned and another vehicle erupted in flames.
One person died at the scene, the agency said.
Southbound traffic was backed up for miles as crews worked to clear the wreckage and an investigation got underway.
Commuters inched past the scene in a single lane for more than an hour before all lanes were ultimately reopened.