The death toll in what police called a horrific human trafficking case -- eight men died inside a sweltering 18-wheeler parked outside a San Antonio Walmart -- reached nine on Sunday.
James Mathew Bradley Jr., 60, from Clearwater, Florida, was taken into custody, federal prosecutors said. No immediate charges were filed.
One additional man died at the hospital, according to Liz Johnson, a spokeswoman for U.S. Immigration and Customs Enforcement. ICE had announced 10 were dead before revising the number to nine.
A total of 39 people had been inside the tractor-trailer, including at least four teenagers, Fox 29 reported. Twenty people were taken to area hospitals in critical condition, and another eight had “less critical” injuries.
"We’re looking at human trafficking crime here this evening,” Police Chief William McManus said, adding that it was "a horrific tragedy."
The U.S. Department of Homeland Security is also looking into the incident.
A store employee first alerted police after being approached by someone from the truck who was asking for water. The vehicle did not have a working air conditioning system when it was found, authorities said.
"They 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 in a news conference. "It was a mass casualty situation for us."
The National Weather Service said the temperature in San Antonio hit 101 degrees just before 5 p.m. Saturday and didn't dip below 90 degrees until after 10 p.m.
According to police, surveillance video from the store showed "a number of vehicles" picking up people who survived the trip that were inside the trailer. Some people fled into the woods, prompting police to search the area in the morning.
The origin of the truck is still unknown. Investigators gathered evidence from the truck on Sunday, which had an Iowa license plate but no other markings.
Thomas Homan, the U.S. Immigration and Customs Enforcement acting director, said in a statement: "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," Homan said in the statement.
U.S. Attorney Richard Durbin, Jr. also called Sunday morning's incident "an alien smuggling venture gone horribly wrong."
"All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo," Durbin said, adding that the Justice Department will be working with Homeland Security and local responders in the investigation.
Other cases of human trafficking in the United States have led to more deaths. In May 2003, 19 immigrants who were being transported from South Texas to Houston inside a sweltering tractor-trailer died.
The Associated Press contributed to this report.