The incident happened around 12:37 p.m. on the westbound 10 near the 405 Freeway interchange.
One person was injured in a FlixBus that was heading to Oakland. A window on the bus was visibly shattered as investigators looked into the incident.
The CHP issued a SigAlert for the transition road.
The passenger, described as a young adult female, suffered minor injuries, possibly from the shattered glass. She was transported to a local hospital.
The transition road was temporarily shut down as the CHP investigated. Investigators weren't immediately sure if the shot came from another vehicle on the freeway or from someone off the freeway.
There were 20 passengers on the bus. Another FlixBus was brought in to transport them.