'It would be premature of me to speculate on what happened at this collision. 'We owe it to the families of those that were killed and injured as well at the public to conduct a complete and thorough investigation'.
Hospital and police officials gave slightly different tallies of the injured and dead in the incident.
Judy Cruz, the managing director of the emergency room department at El Centro Regional Medical Center, said the vehicle had 28 occupants and that 15 had died.
But Watson said at a morning briefing there were 25 occupants in the Expedition and that 13 died, including the driver of the SUV.
A 16-year-old was among the injured, the CHP said, with the oldest victim aged 55.
"Other people were found deceased within the vehicle, and some of the wounded were able to pull themselves out of the vehicle", Watson said during a news conference. People come back and forth on a daily basis for work.
Ms Cruz said: "We believe there was 27 passengers in this SUV that struck a semi-truck full of gravel". The driver of the gravel truck sustained only moderate injuries, Watson said.
It is "too early in the investigation to say what they were doing or where they were coming from", Watson said. It was a sunny, clear morning and authorities said the tractor-trailer and its two empty containers were northbound on State Highway 115 when the SUV pulled in front of it from Norrish Road.
According to Watson, the accident occurred when the SUV entered an intersection in front of the semi-truck, although it was not known whether the smaller vehicle had stopped at the stop sign.
Some of the people who died had been ejected from the SUV by the force of the impact, Watson said. Local hospital officials initially said 15 people died from the crash, but the California Highway Patrol later lowered the figure.
The cause of the collision was undetermined, authorities said, and it also was unknown why so many people were crammed into a vehicle built to hold eight people safely.
Separately, a series of harrowing collisions involving vehicles carrying farmworkers en route to the state's Central Valley agricultural region led to the passage of legislation requiring vans and buses carrying nine or more workers to have seat belts and undergo annual safety certification, the newspaper said.
Shortly before the Highway Patrol news conference, officials from the El Centro Regional Medical Center, which received numerous people injured in the crash, said that 28 people had been in the SUV. Most of the dead were Mexicans, a Mexican official said. In addition, four patients were taken to another medical center while three were transferred to Pioneers Memorial Hospital.
"We had 12 fatalities on scene, one person passed over at the hospital, a total of 13 people passed away", he said.
U.S. authorities are now working with the Mexican Consulate to determine who was in the SUV and notify their next of kin.
Officials with the Imperial County Fire Department told ABC7 that the crash resulted in a second-alarm response, meaning that there were "25 additional personnel called to the scene along with at least four additional ambulances and four air helicopters".
The National Transportation Safety Board also said it will investigate the crash. He said the immigration status of those in the vehicle was unknown and was still under investigation by the California Highway Patrol.