Along with the death and the 20 injured, Border Patrol agents at the scene, who asked not to be identified, said there may have been three others in the Bronco who fled south toward Mexico and one who may have run north.
Agents said they spotted foot tracks heading away from the crash site.
Agents found the injured strewn around the Bronco, which they said is designed to carry five to six people.
The crash was deemed a mutual aid alert, with crews from Calexico, El Centro, Brawley, Calipatria and the county fire departments called to the scene along with Gold Cross ambulance crews.
The Highway Patrol, the Border Patrol and the county Coroner's Office also were at the scene.
El Centro Fire Department Battalion Chief Bill DuBois said about a half dozen victims suffered severe injuries, another half dozen had serious injuries and the rest had minor injuries.
Ambulances could be seen driving down Highway 98 and Interstate 8 as they raced toward El Centro Regional Medical Center and Pioneers Memorial Hospital in Brawley.
At the scene rescue workers formed a triage-like operation, preparing the injured for transport. The injured were lying in rows, waiting to be taken to hospitals.
California Highway Patrol Officer Darren Myers said the accident occurred because the driver was driving with the headlights off on a dark dirt road.
He said the vehicle was driving at moderate speeds and there was no information the group was fleeing Border Patrol agents. He said the driver likely failed to negotiate a curve in the road, causing the vehicle to overturn.
There were few details on the person who died other than it was a man thought to be in his late 30s or early 40s.
Border Patrol agents said they were unsure if there was a smuggler in the group. They speculated if three people fled south it is possible a smuggler was among those people.
One agent said of smugglers, "They have a total disregard for human life. No rational person would pack a vehicle with that many people. It is sad. These people put all their trust into these animals."
There were no details as to whether the Bronco was a stolen vehicle. There was information the vehicle for a time had been in San Jose.
Border Patrol agents said they planned to interview the victims to determine more details and to see if there was a smuggler in the group.
Border Patrol Agent Manuel Figueroa said the death brought the immigrant death toll for the Border Patrol's fiscal year to 89. The fiscal year ends Sept. 30.
He said this summer there has been a successful effort to prosecute smugglers, including pending cases against four smugglers in connection with two desert deaths.
Agents said a smuggler convicted of causing a death can face life imprisonment.
"We hope they know that," Figueroa said. "We hope it does discourage them."
Staff Writer Darren Simon can be reached at 337-4082.