Nine of the people found crammed inside a tractor-trailer outside a Walmart store in San Antonio, Texas, have now died, police announced on Monday 24 July.
The victims were discovered after police received a phone call from a Walmart employee alerting them.
Emergency workers rescued a further thirty people from the tractor-trailer, 17 of whom are being treated for life-threatening injuries.
The survivors are suffering from extreme heatstroke and dehydration, police said. The temperature in San Antonio over the weekend was stifling, reaching 38 degrees on Saturday.
The people in the trailer 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 video posted on Facebook, Hood said that officers were treating the incident as a "mass casualty situation" and had responded in the same way they would in the case of a plane disaster.
Police say that the people inside the truck are victims of human trafficking. San Antonio is a few hours drive from the Mexican border.
"We're looking at a human-trafficking crime here," San Antonio Police Chief William McManus said. "Homicide will work to determine the origin of this horrific tragedy."
The driver of the truck, who has not been identified, has been arrested and is expected to face both state and federal charges. The suspect is due in court on 24 July.