California Highway Patrol Border Division Chief Jim Abele says the early Sunday crash just north of Palm Springs injured an additional 31 people.
He says the USA Holiday tour bus was returning to Los Angeles after a visit to the Red Earth Casino in Thermal, California.
He says the bus was traveling much faster than the semi, and the impact was so strong, the bus went about 15 feet into the trailer. He says it's not clear if the bus was speeding.
Abele says it's unknown if alcohol, drugs or fatigue played a role in the accident. The bus was inspected in 2014, 2015 and this year and found no mechanical issues.
California Highway Patrol Officer Stephanie Hamilton tells the Desert Sun newspaper in Palm Springs that the driver was an owner of USA Holiday, based near Los Angeles. Authorities say the bus may have been coming from a casino.
The Federal Motor Carrier Safety Administration says the company has one vehicle and one driver. Records show it had no crashes and one inspection in the two years before Oct. 22 and had a satisfactory safety rating.
A phone message left for the company wasn't immediately returned. A Facebook message from USA Holiday said it didn't have much information about the crash.
The company posts on social media about trips leaving the Los Angeles area to casinos around the Coachella Valley and Las Vegas.
CHP Officer Stephanie Hamilton confirmed the fatalities to the Desert Sun newspaper in Palm Springs as of 7:30 a.m. Sunday. The newspaper reports more bodies are being removed by firefighters using ladders to climb into the bus' windows.
It says at least 25 bus passengers have been taken to area hospitals.
Desert Regional Medical Center in Palm Springs has received 14 patients. Hospital director of marketing Rich Ramhoff says five are in critical condition, three in serious condition and six had minor injuries.
Lee Rice of Eisenhower Medical Center says the Rancho Mirage hospital received 11 adult patients, all with minor injuries.