The truck was found overnight in a Walmart parking lot and authorities described the incident as a "human trafficking crime."
During a press conference early Sunday morning, local police and fire officials explained the situation and confirmed the U.S. Department of Homeland Security Immigration and Customs Enforcement were called in to investigate.
Seventeen of the 30 injured were transported to local hospitals with serious condition and the other 13 individuals sustained less severe injuries. Many of them suffered from heat stroke and dehydration.
Authorities said the group included adults in their 20s and 30s, and at least two young children.
The driver of the truck has been arrested and could face federal and state charges.
San Antonio Police Chief William McManus said a Walmart employee contacted the police for a welfare check after someone from the truck entered the store asking for water.
"We're looking at a human trafficking crime here," McManus said.
San Antonio Fire Chief Charles Hood said firefighters arrived at the scene at 12:26 a.m.and started removing individuals from the truck, which had no working air conditioning. Hood also said all of the deceased were adult males.
Authorities do not know how long the truck was parked at the location for, but surveillance footage shows multiple cars entering the parking lot and "picked up lots of folks that were in that trailer that survived."
McManus called it a "horrific" tragedy, but said the discovery "is not an isolated incident."
He added, "This happens quite frequently ... fortunately there are people who survived, but this happens all the time."
Photo Credit: Twitter / @wenz_epa