According to Dimmit County Sheriff Marion Boy, the vehicle was traveling around 100 miles per hour when the crash happened off Highway 85, WOAI reported.
The Texas Department of Public Safety said it was investigating the crash, which occurred at 11:18 a.m. local time outside of Big Wells, Texas. The sheriff told the San Antonio NBC affiliate that it was "good police work" that led to deputies being involved in the chase.
An SUV carrying 14 people rolled over and ejected 12 people who are believed to be illegal immigrants, according to a report from News4SanAntonio.
Representatives for both U.S. Customs and Border Protection and the Dimmit County Sheriff's Office did not immediately respond to HuffPost's request for comment. The driver, believed to be a United States citizen, was rushed to a hospital; the passenger, also believed to be a US citizen, now is in custody.
The chase started when a border control agent spotted three vehicles traveling together and became suspicious of smuggling.
Four of those passengers were pronounced dead at the scene and another at the hospital.
"It's just extremely busy, we need the assets down here, we have a problem", Boyd said.
In a later interview, Boyd said the vehicle reached 100 miles per hour before the driver lost control and crashed. "We've seen this many, many times, in not only this county but other counties along the border". "This is a flawless example of why our borders need to be secure".
According to WOAI 5 people were transported to San Antonio Military Medical Center.