A teen who said he was aboard the bus recounted hearing a "loud boom."
Steven Clavijo, 18, who attends West Ranch High School in Santa Clarita, Calif., told The Associated Press that he was on the bus during the deadly crash.
For hours during the long drive north, the students — from several different schools in the Los Angeles area — watched movies and listened to hip-hop on the radio, according to Clavijo.
But just as he was trying to get some sleep in his seat in the back of the bus, he said he felt the vehicle shake from left to right.
"I just heard this loud boom," he said. "We knew we were in major trouble."
Many escaped through a window that someone had kicked open, running for their lives to the other side of the freeway before hearing an explosion and seeing the bus burst into flame.
The bus was one of two that the admissions office at Humboldt State University had chartered to bring prospective students from Southern California to tour the Arcata campus, Humboldt's Vice President of Administrative Affairs Joyce Lopes said.