At least two people are dead in a "major" crash involving a school bus that occurred on a Texas highway Friday, according to a law enforcement official.

A passenger vehicle and a concrete truck were also involved in the crash, according to the Texas Department of Public Safety.

The incident occurred on State Highway 21 West at Caldwell Road, according to the Bastrop County Sheriff's Office.

The "major traffic event" involved an out-of-county school bus, the sheriff's office said.

More than 40 people are "involved" in the crash, according to Austin-Travis County EMS. Four patients were being transported via helicopter from the scene, it said.