At least 13 persons were killed and 31 injured after a tour bus and big rig collided in Southern California on Sunday morning, local authorities said.The tour bus had crashed into the big rig's trailer near Palm Springs, about 100 miles east of Los Angeles. Photos from the scene showed the front end of the bus had been demolished, Xinhua news agency reported.The tour bus was identified as a USA Holiday bus. The Los Angeles-based firm typically runs from LA to Southern California.Firefighters were using ladders placed near the bus' windows to pull out victims. Rescuers continued to search the wreckage for more victims. Throughout the morning, more bodies were removed from the bus and placed along the road.Authorities shut down the freeway following the crash. Details on the cause of the crash were unclear.