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A Florida truck driver was in custody Sunday after nine people were found dead in the back of a cramped, overheated 18-wheeler in San Antonio, Texas.

More than a dozen other people — whom authorities described as victims of a "horrific" human smuggling operation — suffered life-threatening injuries.

In a statement, the U.S. attorney's office for western Texas said the driver, James Mathew Bradley Jr., 60, of Clearwater, Fla., was in custody pending criminal charges. A complaint will likely be filed Monday morning, the statement said.

A Walmart security guard in a southwest section of the city made the discovery after a tipster identified a tractor-trailer in the parking lot that was apparently full of migrants, said Joe Arrington, a spokesman for the San Antonio Fire Department.

The tipster, who was not identified, had been in the truck and approached the security guard to ask for water, San Antonio Police Chief William McManus later told reporters.

The security guard found the dead and sick when he searched the back of the truck, Arrington said. A total of 39 people were inside, the U.S. attorney's office said Sunday afternoon. Officials reported earlier that 38 people were found in the trailer, but they said later that they had found an additional person in a wooded area nearby.

"The truck was loaded with people," Fire Chief Charles Hood told reporters.

Eight people were initially found dead in the tractor-trailer, and an additional victim died at a hospital, a spokeswoman for Immigration and Customs Enforcement told NBC News.

All of the dead are adult men, the U.S. attorney's office said, and 30 others were being treated at hospitals.

ICE had said earlier that two people died at hospitals, but it later revised the number, citing miscommunication with hospital officials.

Officials investigate a truck that was found to contain 38 suspected migrants in San Antonio, Texas, on Sunday. Darren Abate / EPA

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Some inside the semi ran into nearby woods, triggering a search by helicopter and on foot, McManus said, adding that police would look for the missing again in the morning.

"We're looking at a human trafficking crime here this evening," he said, describing it as a "horrific tragedy." He added that the Department of Homeland Security was working with local police.

After the victims are treated, they will be investigated by ICE, McManus said.

First responders raced to the scene shortly after 12:30 a.m. (1:30 a.m. ET), officials said. Hood said that the people in the truck were "very hot to the touch" and that there were no signs of water inside. The air conditioning was not working, he said.

"Our paramedics and firefighters found that each one of them had heart rates over about 130 beats per minute," he said. "You're looking at a lot of heat stroke, a lot of dehydration."

San Antonio police said in a statement later Sunday that all of the dead were believed to have succumbed to heat exposure and asphyxiation. Official causes of death will be determined by the Bexar County medical examiner.

Police added that they do not yet know the exact country of origin, destination or demographics of the dead and injured, although Mexico's consul general, Rayna Torres, confirmed Sunday that Mexican nationals were among them.

Citing the U.S. law enforcement investigation, Torres said she did not want to provide specifics, but she said that some were minors. Some could not speak, she added, because they are in grave condition.

Police said that the two youngest known victims, both of whom survived, were 15 years old.

San Antonio, Texas, police investigate a truck in side which eight people were found dead Sunday outside a Walmart store in stifling summer heat. Eric Gay / AP

The National Weather Service said the temperature in San Antonio hit 101 degrees on Saturday and didn't dip below 90 degrees until after 10 p.m., according to The Associated Press.

Closed-circuit TV images from before emergency services arrived showed several cars turn up to pick up many of those who had survived the journey inside the truck, McManus said.

The driver and anyone else involved in the incident will face state and federal charges, McManus said.

"This is not an isolated incident," he said, as he urged anyone who sees anything similar to call 911. "Fortunately, we came across this one. Fortunately, there are people who survived. But this happens all the time."

Acting ICE Director Thomas Homan said in a statement Sunday that smuggling networks "have repeatedly shown a reckless disregard for those they smuggle, as last night's case demonstrates."

"By any standard, the horrific crime uncovered last night ranks as a stark reminder of why human smuggling networks must be pursued, caught and punished," he said.

Rep. Joaquin Castro, D-Texas, said the deaths were "tragic and avoidable."

My statement on the tragic deaths of eight immigrants in San Antonio this morning: pic.twitter.com/MBceh1RBtu — Joaquin Castro (@JoaquinCastrotx) July 23, 2017

Texas Gov. Greg Abbott said in a statement that Texas is "working to eradicate" traffickers, while Jonathan Ryan, executive director of the Refugee and Immigrant Center for Education and Legal Services, said the political environment was partly to blame.

"You can draw a direct line between the hostile policies and rhetoric against immigrants that are happening nationally, and here in our state, to events like what happened today," Ryan said.

"You can change laws. You can change policies," he said. "But you cannot change the fact that people fleeing violence, people seeking to save and protect their families, are going to do whatever they can to flee that danger and find safety."

Saphora Smith reported from London. Dystany Muse, Kurt Chirbas and Daniella Silva reported from New York.