The incident was first reported shortly before 5:30 a.m. along the westbound 60 Freeway near the Phillips Ranch Road exit.
According to the California Highway Patrol, it all started with an accident, a possible hit-and-run. Then, a man pulled a gun on another man.
It was not known if any shots were fired, but authorities confirmed that at least one man was injured, and one man was arrested.
The scene was off to the shoulder, but westbound traffic was backed up for miles.