The survivors in the tractor-trailer parked by a Walmart store in San Antonio, Texas, are suffering from extreme heat stroke and dehydration, police have said.
Nine people were found dead inside the trailer truck early on Sunday morning (23 July) after police received a phone call from a Walmart employee alerting them.
A further 30 inside the truck were injured, 20 of whom are reported to be in critical condition.
Police believe the people in the tractor-trailer are victims of human trafficking and are hunting those responsible.
San Antonio Fire Chief Charles Hood said that paramedics and firefighters who treated the victims found that they all had accelerated heartbeats caused by heat stroke and severe dehydration.
The temperature in San Antonio over the weekend was stifling, reaching 38 degrees on Saturday.
"They were very hot to the touch. So these people were in this trailer without any signs of any type of water," Hood told AP.
In a video posted on Facebook, Hood said that officers were treating the incident as a "mass casualty situation" and were responding in the same way they would to an airplane disaster.
"We are very fortunate that there weren't 38 people who were locked inside of this vehicle dead," he said.
The driver of the truck, who has not been identified, is in custody and is expected to face both state and federal charges, San Antonio Police Chief William McManus said earlier.
"We're looking at a human-trafficking crime here," he said. "Homicide will work to determine the origin of this horrific tragedy."
San Antonio is a few hours drive from the Mexican border. The US immigration department is working to establish the victims' status.
The Department of Homeland Security are assisting in the investigation.