At least 11 people died and another 12 were injured after their pickup truck left the highway and crashed into trees in rural South Texas on Sunday, authorities said.

"This is the most people I've seen in any passenger vehicle, and I've been an officer for 38 years," said Gerald Bryant, a spokesman for the Texas Department of Public Safety.

At least two young children were among the dead, Bryant said. The 23 people were loaded inside both the truck's cab and bed.

The pickup was heading north on U.S. 59 on Sunday evening when it traveled off the right side of the highway near the unincorporated community of Berclair and struck two large trees, Bryant said.

He said several surviving victims had life-threatening injuries that he described as "very serious."

The U.S. Border Patrol will assist with the investigation.

"It's unknown whether or not (the victims) were illegal, but it's possible," Bryant said.