Sixteen people who were found alive, including 12 adults and four children, are now recovering in hospital.
Mexico’s Foreign Minister Marcelo Ebrard confirmed two of the survivors were from Guatemala.
The truck was found next to railway tracks in a remote area of San Antonio.
San Antonio Police Chief William McManus said a worker nearby heard a cry for help around 5pm Texas time and went to investigate.
“(They) found a trailer with the doors partially open and opened them up to take a look and found a number of deceased individuals inside,” McManus said.
Hood said a body was found outside the truck and the remainder were found inside the trailer.