A 16-year-old boy has died after striking a freeway overpass in California while standing up on a double-decker Starline Bus.

According to NBC Los Angeles, the incident happened on Thursday night while the bus was traveling at 45 miles per hour on the San Diego Freeway. A witness told NBC that the teenager, Mason Zisette, may have been standing on his seat:

A witness told NBC4 that the teen may have been dancing on a seat cushion with his back to the freeway signs when his head and hands slammed into the sign. He then collapsed on the floor, the witness said.

About two dozen teens—most under 18 years old—were dancing on the upper level when the incident occurred, while all of the adult chaperones were inside the bus. Zisette was rushed to the hospital and listed in critical condition. He died two days later.

The California Highway Patrol is investigating the incident.

[image via NBC Los Angeles]