(CNN) At least nine people are dead after a phone call from a Walmart employee led to the discovery of dozens of undocumented immigrants severely injured in the back of a tractor-trailer in Texas early Sunday, according to federal authorities.

More than 100 undocumented immigrants may have been sandwiched in the back of the 18-wheeler during the journey, US Immigration and Customs Enforcement acting Director Thomas Homan said, citing initial interviews with survivors.

"Checking the video from the store, we found there were a number of vehicles that came in and picked up a lot of the folks that were in that trailer that survived the trip," San Antonio Police Chief William McManus said. "We are looking at a human trafficking crime here."

The employee told authorities the trailer was parked at a Walmart lot in San Antonio. A man who was in it asked the employee for water.

The employee brought water for the man, then called police and asked them to conduct a welfare check, McManus said.

That's when authorities discovered eight bodies and 31 people suffering from various injuries, authorities said. A ninth victim later died at a hospital, ICE officials said.

San Antonio officers investigate the scene where people were found dead in a tractor-trailer outside a Walmart.

"We quickly called a 'mass casualty incident' and had about 29 units arrive out there and start transporting people," San Antonio Fire Chief Charles Hood said.

"With heat strokes or heat injuries, a lot of them are going to have some irreversible brain damage."

A heatstroke can cause swelling of the brain and other vital organs, possibly causing permanent damage, if a person's body temperature isn't quickly lowered, according to the Mayo Clinic

Officials said the air conditioner in the trailer was not working. And the high temperature in San Antonio on Saturday was 100 degrees Fahrenheit (38 degrees Celsius).

"Unfortunately, some of them were severely overheated, and that was a refrigerated truck with no refrigeration," Hood said. "So the inside of the truck was just austere condition that nobody was going to survive in it. So we were very fortunate that they were found."

Two of the people hospitalized are 15 years old, the fire spokesman said.

Feds join the investigation

Authorities don't know where the trailer is from or how long it was parked at Walmart, but they are reviewing surveillance video.

Police also searched the area with helicopters after some people ran into the woods.

JUST WATCHED Fire chief: Truck had austere conditions Replay More Videos ... MUST WATCH Fire chief: Truck had austere conditions 00:47

The US Attorney's Office said the driver, James Matthew Bradley Jr., 60, of Clearwater, Florida, was being held in connection with the incident, authorities said. Prosecutors plan to file a criminal complaint against Bradley in federal court on Monday morning.

"The driver and whoever else we find is involved in this will be facing state and federal charges," McManus said.

"Fortunately, we came across this one and fortunately there are people who survived but this happens all the time," he said. "You can see that it happens late at night under darkness because they don't want to be discovered."

Once the victims undergo treatment, the case will be turned over to US Immigration and Customs Enforcement, the police chief said.

Homan deplored the discovery as the latest major case of human smuggling in the United States.

"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," Homan said in a statement.

He said in fiscal year 2016, Homeland Security Investigations launched 2,110 human smuggling investigations, which resulted in 1,522 criminal convictions.

During that same year, he said, HSI made 2,734 criminal arrests and 3,007 administrative arrests related to human smuggling operations.

In Sunday's case, "These people were helpless in the hands of their transporters," said Richard L. Durbin Jr., US attorney for the Western District of Texas.

"All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo."

US Rep. Lloyd Doggett, a Democrat who represents parts of San Antonio, called for reform.

"Horrific scene overnight in San Antonio where a human smuggling attempt turned deadly," Doggett tweeted . "Prosecute smugglers, pray for survivors and the victims' families, stop the hysteria, reform our broken immigration system."

Texas Gov. Greg Abbott said the deaths were "a heartbreaking tragedy."

"Human trafficking is an epidemic that Texas is working to eradicate," the governor said, highlighting a bill he signed in May that provides information to the trucking industry about how to spot and report signs of human trafficking.

My heart goes out to the victim's families. Our human trafficking unit will provide any resources needed to help w/the investigation. — TX AG's Office (@TXAG) July 23, 2017

Walmart: Some of the incident was on camera

The store captured some of the incident on camera, but declined to describe or release any of the footage to CNN, Walmart corporate spokesman Nathan Barr said.

Walmart has declined to publicize details of what happened, citing the investigation.

"(T)his is an ongoing law enforcement investigation. We are cooperating fully with the authorities but will not be providing interviews or footage to media," Barr told CNN in an email.

"We are in the process of reviewing that video with law enforcement but won't be commenting on it."

Barr also declined to identify the employee who asked police to conduct a welfare check on the truck.

"I wish I could be more helpful but we can't interfere with the investigation," he said.