"The information that we have is that the bus' brakes failed and the accident occurred," he said.
Investigators had not yet obtained the passenger list from inside the bus.
InterBus Tours offered condolences to all those affected through a statement posted on its Facebook page, which also had a posting from last week advertising the trip to Big Bear, a lakeside recreation area at an elevation of 6,750 feet in the San Bernardino Mountains.
"InterBus Tours and its entire team are working to support our customers and their families who unfortunately suffered an accident in the mountains of Big Bear in San Bernardino. Personnel from InterBus are in our offices as well as in hospitals where we know authorities have taken the injured," the statement said.
It noted the vehicle was subleased and has insurance to cover such an accident.
After the crash, it took nearly two hours to clear all the people who could be transported to hospitals and go through the wreckage, said Kathleen Opliger, incident commander and San Bernardino County fire battalion chief.
Route 38 runs through San Bernardino National Forest to Big Bear. The accident occurred as the bus was headed south and leaving the forest.
"It happened so fast I don't know how it all happened," a passenger who declined to give her name told the San Bernardino Sun (http://bit.ly/XiWWdu). "This was supposed to be a good day out with my companions and then this happened."
Patients were taken to several hospitals with injuries ranging from minor to life-threatening.
The National Transportation Safety Board was sending a team to the scene.
The California crash came less than a day after a bus carrying 42 high school students and their chaperones slammed into an overpass in Boston. Massachusetts state police said 35 people were injured after the driver had directed the bus onto a road with a height limit.
Associated Press writers contributing to this report included Michael R. Blood, Andrew Dalton and Bob Jablon in Los Angeles, Amanda Kwan and Bob Seavey in Phoenix