Merced County Sheriff Vern Warnke tells The Associated Press that the bus hit the pole of a highway sign head-on early Tuesday, and it sheared through the vehicle before stopping at the first axle "with a great impact."
PHOTOS: Deadly tour bus crash on California's Highway 99
Warnke says rescuers brought out "bags of body parts" from the survivors following the crash on State Route 99 just south of Livingston.
The sheriff says the bus operated by Autobuses Coordinados USA was heading to Washington state and was due in Livingston at 1:30 a.m. to change drivers.
The California Highway Patrol says five people died and at least five people were airlifted to hospitals in unknown condition.
The cause of the crash is still unknnow.
One northbound lane of the freeway has been reopened. Atwater is a city of 28,000 people in Merced County, about 65 miles northwest of Fresno.
MORE: Driver ID'd as: Mario David Vasquez, 57 - still alive but major injuries. Hospitalized. Right now 5 dead, 6 airlifted @abc7newsbayarea— Jessica Castro (@JessicaABC7) August 2, 2016