Ten are dead after a FedEx truck crossed over a grassy median on a Northern California freeway and slammed into a tour bus carrying more than 40 high school students Thursday evening, authorities say.

The students were headed on a college visit to tour Humboldt State University. One student, Steven Clavijo, 18, was trying to take a nap when he felt the bus shaking from left to right before hearing a loud boom.

Both the driver of the bus and the semi were among those killed.

Clavijo said that someone kicked one of the windows open and many escaped that way, running to the other side of Interstate 5. Many escaped just before hearing an explosion and seeing the bus burst into flanes.

Huge flames could be seen coming from both vehicles and black clouds of smoke entered the air before firefighters were able to put out the fire.

Inside the charred bus, bodies were draped in blankets. Among the dead were three adult chaperones and five students.

An investigation team from the National Transportation Safety Board is expected to arrive Friday.

Humboldt State University had chartered two buses from various Southern California high schools to tour their campus in Arcata, Calif.

Silverado Stages, who owns the bus involved in the crash, is assisting authorities with the investigation.

Several different schools in the Los Angeles County Unified School District were among those invited on the tour of the college. It was a scheduled three day visit.

The California Highway Patrol says that the bus and the truck were on opposite sides of the freeway when the truck allegedly tried to avoid collision with a white sedan. It hit the car on the side before heading across the grassy median and into the bus. No one in the white sedan was injured.

In addition to the deceased, a first responder on the scene noted that there were 36 or 37 injuries.

"The victims were teenage kids. A lot of them were freaked out. They were shocked. They still couldn't grasp what happened," Jason Wyman of the Orland Volunteer Fire Department said.

At least four people are listed in critical condition at various hospitals.

FedEx is cooperating with authorities according to a statement from a spokesperson.