The death toll in a suspected immigrant-smuggling attempt gone wrong in Texas has risen to 9 after two people died in hospital, officials said.
Eight people were earlier found dead in the back of a tractor-trailer in the car park of a Walmart store in San Antonio.
Thomas Homan, acting director of US Immigration and Customs Enforcement, said that based on initial interviews with survivors, there may have been more than 100 people in the truck.
Thirty-eight were found inside, and the rest are believed to have fled or been picked up.
Mr Homan said some survivors have identified themselves as Mexican nationals.
Four of the passengers are believed to be between 10 and 17 years old, and at least one of them is in serious condition.
The driver was arrested and nearly 20 others rescued from the lorry were taken to hospital in a serious condition, many with extreme dehydration and heatstroke.
"We're looking at a human-trafficking crime," police chief William McManus said, adding that many of those inside the 18-wheel vehicle appeared to be in their 20s and 30s and that there were also apparently two school-age children.
He called it "a horrific tragedy".
Authorities did not say whether the truck was locked when they arrived, whether it was used to smuggle the occupants across the border into the US, or where it might have been headed. San Antonio is about a 150-mile drive from the Mexican border.
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.
The temperature in San Antonio reached 38C on Saturday and did not dip below 32C until after 10pm. The trailer did not have a working air conditioning system, Mr Hood said.
The tragedy came to light after a person from the truck approached a Walmart employee in the car park and asked for water late on Saturday night or early Sunday morning, Mr McManus said.
The employee gave the person water and then called police, who found the dead and the desperate inside the truck. Some of those in the truck ran into the woods, leading to a search, Mr McManus said.
Hours later, after daybreak, a helicopter hovered over the area, and investigators were still gathering evidence from the tractor-trailer, which had an Iowa licence plate and was registered to Pyle Transportation of Schaller, Iowa.
Investigators checked store surveillance video, which showed vehicles arriving and picking up people from the truck, authorities said.
"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," Mr Homan said.