El CENTRO, Calif. (AP) — A semitruck on Tuesday crashed into an SUV carrying 25 people on a Southern California highway, killing 13 and leaving others injured, authorities said.

California Highway Patrol Chief Omar Watson said 12 people died at the scene, which is about 11 miles (18 kilometers) north of the U.S.-Mexico border, and another died after arriving at the hospital. Hospital officials earlier reported there were 15 killed and more people in the SUV.

Highway Patrol said it's not clear what caused the crash between the SUV and a tractor-trailer full of gravel on a highway through fields in the agricultural southeastern corner of California, about 100 miles (160 kilometers) east of San Diego.

It's unclear if the SUV had stopped at a stop sign before crossing into the path of the semitruck around 6:15 a.m., Watson said. The truck struck the left side of the SUV, which appeared to have been pushed off the road.

“Obviously, that vehicle’s not meant for that many people,” Watson said. “It’s unfortunate that that many people were put into that vehicle because there’s not enough safety constraints to safely keep those people in that vehicle.”

He said numerous people were ejected. The SUV was a Ford Expedition that would typically seat eight to nine people legally.

Multiple people from the SUV were flown or sent to hospitals for injuries that included fractures and head trauma. Four were flown to Desert Regional Medical Center in Palm Springs, where is in critical condition and the others are stable, spokesman Todd Burke said. The truck driver was hospitalized with moderate injuries.

Macario Mora, a spokesman for Customs and Border Protection in Yuma and El Centro, said the Border Patrol was helping other law enforcement with the crash. He said the immigration status of those in the SUV was unknown and being investigated.

“It was an unusual number of people in an SUV, but we don’t know who they were,” Mora said, adding that they could have been farmworkers.

A harvest is underway in the region of most of the winter lettuce and other leafy greens eaten in the United States.

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.