Boyd said there were 14 people in the vehicle. Authorities believe the driver and one passenger are US citizens.
Local law enforcement were familiar with the driver, who was in the area last week to either pick up undocumented immigrants or the scout, the sheriff said.
The other 12 passengers were undocumented immigrants, Boyd said.
Boyd said the victims were taken to local hospitals. Six patients were taken to San Antonio Military Medical Center in San Antonio, Lt. Edwin Roberts said. He would not confirm if any of the patients had died.
The sheriff said he didn’t know where the immigrants were from but said most migrants who travel through the area are from Central America or Mexico.
He said his deputies are often involved in chases pursuing people suspected of smuggling undocumented immigrants and drugs.
Boyd added: “If it can be built, I think it needs to be built. But along with that, there needs to be cameras. There needs to be sensors.”