Another 28 people were found in the trailer on Sunday, which lacked air conditioning or a water supply; many in critical condition and suffering from heat stroke and exhaustion.
A tenth person died on Monday after 38 people – eight dead – were found on Sunday inside a sweltering tractor trailer parked at a Walmart store in San Antonio, Texas, and authorities said they were the victims of "ruthless" human traffickers.
Another 28 people, many in critical condition and suffering from heat stroke and exhaustion, were also in the trailer, which lacked air conditioning or a water supply, San Antonio Fire Chief Charles Hood said. Temperatures outside the vehicle topped 100 degrees Fahrenheit (37.8 Celsius).
The truck's driver was arrested and will face charges, said Richard Durbin, US Attorney for the Western District of Texas, and prosecutors are working to identify others responsible.
The bodies were discovered after officials were led to the trailer by a man who had approached a Walmart employee and asked for water.
"All were victims of ruthless human smugglers indifferent to the well-being of their fragile cargo," Durbin said.
"These people were helpless in the hands of their transporters. Imagine their suffering, trapped in a stifling trailer in 100-plus degree heat," he said.
TRT World's Paul Scott reports.
San Antonio is about 150 miles (240 km) north of the border with Mexico. Temperatures in the area held above 100 degrees Fahrenheit until 6 pm local time on Saturday and were expected to soar into the 100s again on Sunday, with humidity making the heat feel close to 110 degrees, forecasters said.
Raids on suspected undocumented immigrants have ramped up across the United States in recent months, after President Donald Trump's vow to crack down on those entering the country without authorization or overstaying their visas.
In Texas alone, federal immigration agents said they arrested 123 undocumented immigrants with criminal records in an eight-day operation that ended last week.
The San Antonio deaths come more than a decade after what is considered the worst immigrant smuggling case in US history, when 70 people were found stuffed into an 18-wheeler. Nineteen of them died in the incident in Victoria, Texas, about 100 miles south-east of San Antonio, in May 2003.
San Antonio Police Chief William McManus described the latest fatalities as a "horrible tragedy" and said other suspects had fled the scene as police officers arrived.
"Checking the video, there were a number of vehicles that came and picked up other people who were in that trailer," McManus said.
Twenty people were airlifted to seven hospitals and their conditions were "critical to very critical," Hood said. Eight others are hospitalised in less serious condition, he said.
At San Antonio's University Hospital, six adults were admitted early Sunday with heat-related injuries, all of them in critical condition, spokesman Donald Finley said.
McManus said the people in the truck ranged from school-age juveniles to adults in their 20s and 30s.
He said the Department of Homeland Security had joined the investigation, and that the origin of the truck is unclear.
Experts have warned in recent months that tougher immigration policies could make it more difficult to stop human trafficking. Measures to harden international borders encourage would-be migrants to turn to smugglers and fear of deportation deters whistle-blowing, they said.
The Border Patrol has regularly reported finding suspected immigrants inside trucks along the US border with Mexico.
Earlier this month, 72 Latin Americans were found in a trailer in Laredo, it said. In June, 44 people were found in the back of tractor trailer in the same Texas city, which lies directly across the Rio Grande from Mexico.
While there are no official law enforcement statistics, nearly 32,000 cases of human trafficking in the United States have been reported to the National Human Trafficking Hotline in the last decade.