At least 13 people are dead and more than 30 injured after an early-morning tour bas crash on I-10 in Palm Springs, California on Sunday.

The bus, which was operated by a Los Angeles-based tour company, slammed into the back of the collapsible trailer of a semi-truck. Because many passengers onboard the bus were not wearing seatbelts, several went flying through the air.

The cause of the crash remains unknown, but investigators said early reports suggest speed may have been a factor.

At least 30 passengers were taken to multiple hospitals. As of Sunday night, five of them were listed in critical condition and seven had been treated and discharged.

The bus driver was among the 13 individuals who died in the crash.

The National Transportation Safety Board will investigate the deadly collision. California Highway Patrol Officers said the bus showed no mechanical issues during its annual inspection in April.