He said the bus was traveling much faster than the semi, and the impact was so strong, the bus went about 15 feet into the trailer.
"The bus company is USA Holiday Bus based in Los Angeles".
Photographs from the scene show the front of the tour bus wedged inside the back of a mangled "big rig" truck trailer.
Witness Poet Aleman, who stopped on the highway to see the grisly aftermath, told The Desert Sun it looked like the truck tried to merge into the lane the bus was travelling in.
Firefighters are using ladders into the bus' windows to pull out victims.
The crash occurred on Interstate 10 near Palm Springs, California, on Sunday morning (overnight Monday NZT) the Desert Sun reported.
As many as 13 people have died in a bus crash along Interstate 10 Sunday morning, it's being reported.
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Lee Rice of Eisenhower Medical Center says the Rancho Mirage hospital received 11 adult patients, all with minor injuries. In August, a bus in central California hit a highway sign post that tore through the vehicle and left four people dead.
Five critically-injured passengers were at the nearest trauma center, at Desert Regional Medical Center in Palm Springs, six miles south of the wreck. The driver was critically injured and taken to a local hospital; per Patch, a fleet of ambulances took the injured passengers to hospitals across the Coachella Valley.
One person was confirmed dead in a tour bus crash on October 23, 2016 in Desert Hot Springs.
According to its Facebook page, USA Holiday has recently made trips to Tortoise Rock Casino, Red Earth Casino and Los Vegas, but the company hasn't promoted any trips on the page since mid-September. "This is a day that will be remembered".
The National Transportation Safety Board said it will investigate the crash and is sending an investigative team to the site.
The crash is among the deadliest in California history.