Texas Department of Public Safety Senior Trooper Gerald Bryant said 23 people were in a white 2000 Ford F-250. Some people were in the passenger compartment and others were in the truck bed.
The pickup was northbound on Highway 59 just north of Berclair when it veered off the road and hit two large trees on the right side, officials said.
No other vehicles were involved in the accident.
According to investigators, 13 of the 23 in the pickup died, 10 others were hurt. Troopers said at least two of the dead were children.
"In my 38 years as an officer, this is one of the worst fatalities I have been to and I have never seen where we had that many in a vehicle," said TX-DOT PIO Gerald Bryant.
The victims who survived were taken to hospitals in Victoria, San Antonio and Corpus Christi. Troopers said the driver survived the crash. They're planning to question the driver to find out what caused the accident.
None of the victims has been identified.
Investigators blocked the roadway for hours due to the investigation. Border Patrol will also be helping with the investigation.
