The driver was arrested, and nearly 20 others rescued from the rig were hospitalized in dire condition, many with extreme dehydration and heatstroke, officials said.
"We're looking at a human-trafficking crime," said San Antonio Police Chief William McManus, calling it "a horrific tragedy."
One U.S. official said Sunday evening that 17 of those rescued were being treated for injuries that were considered life-threatening. The official spoke to The Associated Press on condition of anonymity because the information has not been publicly released.
Authorities were called to the San Antonio parking lot late Saturday or early Sunday and found eight people dead inside the truck. A ninth victim died at the hospital, said Liz Johnson, spokeswoman for U.S. Immigration and Customs Enforcement.
The victims "were very hot to the touch. So these people were in this trailer without any signs of any type of water," San Antonio Fire Chief Charles Hood said.
Authorities would not say whether the trailer was locked when they arrived, but they said it had no working air conditioning.
It was just the latest smuggling-by-truck operation to end in tragedy. In one of the worst cases on record in the U.S., 19 immigrants locked inside a stifling rig died in Victoria, Texas, in 2003.
Based on initial interviews with survivors of the San Antonio tragedy, more than 100 people may have been packed into the back of the 18-wheeler at one point in its journey, ICE acting Director Thomas Homan said. Officials said 39 people were inside when rescuers arrived, and the rest were believed to have escaped or hitched rides to their next destination.
Some of the survivors told authorities they were from Mexico, and four appeared to be between 10 and 17 years old, Homan said. Investigators gave no details on where the rig began its journey or where it was headed.
But Homan said it was unlikely the truck was used to carry the immigrants across the border into the United States. He said people from Latin America who rely on smuggling networks typically cross the border on foot and are then picked up by a driver.
"Even though they have the driver in custody, I can guarantee you there's going to be many more people we're looking for to prosecute," Homan said.
Mexican Consul General in San Antonio Reyna Torres said Mexican nationals were among the survivors and those who died on the rig. The consulate has been in contact with family members both in Mexico and the U.S., Torres said.
The Mexican government also released a statement Sunday evening expressing its condolences to the relatives of those who died and called for an "exhaustive investigation."
Guatemala's foreign ministry added that at least two Guatemalans were on the abandoned tractor-trailer.
Tekandi Paniagua, communications director for the foreign ministry, said the two male survivors told consulate officials they crossed the border by foot at Laredo and boarded the rig. They told officials their final destination was Houston.
Federal prosecutors said James Mathew Bradley Jr., 60, of Clearwater, Florida, was taken into custody and would be charged on Monday. The local U.S. Attorney's Office wouldn't say whether Bradley was the alleged driver of the truck who was arrested. It was not immediately known whether Bradley had an attorney who could speak on his behalf.
The U.S. Homeland Security Department stepped in to take the lead in the investigation from San Antonio police. Department Secretary John Kelly said the incident demonstrates the brutality of smuggling organizations that "have no regard for human life and seek only profits."
The truck had an Iowa license plate and was registered to Pyle Transportation Inc. of Schaller, Iowa. A company official did not immediately respond to a phone message seeking comment.
San Antonio is about a 150-mile (240-kilometer) drive from the Mexican border. The temperature in San Antonio reached 101 degrees (38 Celsius) on Saturday and didn't dip below 90 degrees (32 C) until after 10 p.m.
The tragedy came to light after a person from the truck approached a Walmart employee in the parking lot and asked for water late Saturday night or early Sunday morning, said McManus, the local police chief.
The employee gave the person water and then called police, who found the dead and the desperate inside the rig. Some of those in the truck ran into the woods, McManus said.
Investigators checked store surveillance video, which showed vehicles arriving and picking up people from the truck, authorities said. Walmart released a brief statement Sunday saying it was doing what it could to help investigators.
On Sunday evening, about 100 people gathered at a San Antonio church for a vigil to mourn those killed.
In the May 2003 case, the immigrants were being taken from South Texas to Houston. Prosecutors said the driver heard them begging and screaming for their lives but refused to free them. The driver was sentenced to nearly 34 years in prison.
"It's sad that 14 years later people are still being smuggled in tractor-trailers, there still isn't water, there still isn't ventilation," Homan said. "These criminal organizations, they're all about making money. They have no regard for human life."
The Border Patrol has reported at least four truck seizures this month in and around Laredo, Texas. On July 7, agents found 72 people crammed into a truck with no means of escape, the agency said. They were from Mexico, Ecuador, Guatemala and El Salvador.
Authorities in Mexico have also made a number of such discoveries over the years.
Last December, they found 110 migrants trapped and suffocating inside a truck after it crashed while speeding in the state of Veracruz. Most were from Central America, and 48 were minors. Some were injured in the crash.
Last October, also in Veracruz state, four migrants suffocated in a truck carrying 55 people.
The Department of Homeland Security and U.S. Immigration and Customs Enforcement released the following statements:
"San Antonio firefighters and police responded to a horrific scene this morning on the southwest side of town. They discovered an alien smuggling venture gone horribly wrong. Eight immigrants were found dead. At least twenty more were in serious condition. All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo. The South Texas heat is punishing this time of year. These people were helpless in the hands of their transporters. Imagine their suffering, trapped in a stifling trailer in 100-plus degree heat. The driver is in custody and will be charged. We will work with the Homeland Security Investigations and the local responders to identify those who were responsible for this tragedy." - Richard L. Durbin, Jr. United States Attorney Western District of Texas
Statement from ICE Acting Director Thomas Homan:
"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. U.S. Immigration and Customs Enforcement's Homeland Security Investigations works year-round to identify, dismantle, and disrupt the transnational criminal networks that smuggle people into and throughout the United States. These networks have repeatedly shown a reckless disregard for those they smuggle, as last night's case demonstrates. I personally worked on a tragic tractor trailer case in Victoria, Texas in 2003 in which 19 people were killed as a result of the smugglers' total indifference to the safety of those smuggled and to the law.
"The men and women of ICE are proud to stand alongside our law enforcement partners, including locally and at the U.S. Department of Justice, to combat these smuggling networks and protect the public and those who would fall victim to their dangerous practices that focus solely on their illicit profits. So long as I lead ICE, there will be an unwavering commitment to use law enforcement assets to put an end to these practices."
Statement from Governor Greg Abbott:
"The loss of these lives is a heartbreaking tragedy. Human trafficking is an epidemic that Texas is working to eradicate. To that end, Texas will continue to provide protection for the victims who have been robbed of their most basic rights, and bring down the full weight of the law for the perpetrators of this despicable crime."
The Associated Press contributed to this report.
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