Judy Cruz, director of El Centro Regional Medical Centre’s emergency department, said 12 people died at the scene – which is about 11 miles north of the US-Mexico border – and another died after arriving.
Hospital officials earlier reported there were 15 killed and more people in the SUV.
Officials believe there were 27 people in an SUV that struck a tractor-trailer full of gravel.
Multiple patients were sent to hospitals for their injuries, including four flown to Desert Regional Medical Centre in Palm Springs, spokesman Todd Burke said.
Three were in intensive care, he said.
“The patients are, of course, going through a little bit of a difficult time, as you can imagine,” said Dr Adolphe Edward, the El Centro hospital’s chief executive officer.
It was not immediately clear what caused the crash in the agricultural south-eastern corner of California on a highway running through fields in the Holtville area, about 100 miles east of San Diego.
Officials from the Holtville and Imperial County fire departments, as well as the California Highway Patrol spokesperson for the area, could not immediately be reached.