KIII-TV, Corpus Christi, Texas, reported the passengers were thought to be illegal immigrants. The broadcaster reported federal agents from Immigration and Custom Enforcement and from Border Patrol were called to assist with the investigation.
The Ford F-250 super-duty pickup left U.S. 59 near Berclair in Goliad County Sunday evening and hit "two very large trees," Texas Department of Public Safety spokesman Gerald Bryant said.
The 10 survivors, including a man who said he was the driver, were injured, the San Antonio Express-News reported.
Eleven of the victims died at the scene and two died later in hospitals, KSAT-TV, San Antonio, reported.
Bryant said children were among the victims.
The cause of the crash was under investigation.