Eight people were found dead in the vehicle in a Walmart car park in San Antonio, Texas, and 31 others were treated. Two later died in hospital.
The driver, named as James Mathew Bradley, 60, could face the death penalty.
He says the immigrants were placed in his trailer while he was distracted.
They were inside the trailer without access to air conditioning or water while outside temperatures hit 38C (100F).
Police say they believe the incident is linked to people smuggling.
Video footage from the store reportedly showed a number of vehicles arriving to pick up some of the survivors. Several others may have managed to escape on foot into the woods nearby.
Immigration and Customs Enforcement acting head Thomas Homan said more than 100 people could have been in the truck at one point.
Mexico's government said it was working closely with US authorities to identify the nationalities of the victims.
San Antonio is a few hours' drive from the border with Mexico, and the US immigration department is trying to establish the victims' legal status.
Officials were brought to the trailer by a man who had approached an employee of the Walmart store and asked for water.
San Antonio police chief William McManus said in a press briefing that the people ranged from school age to in their 30s.
Local fire chief Charles Hood said the survivors had heart rates of over 130 beats per minute and were very hot to touch.
The fire chief confirmed at least two of the victims were school-age children. Their condition is not clear.
The US attorney for the Western District of Texas, Richard Durbin, said the people were helpless victims of "ruthless human smugglers indifferent to the wellbeing of their fragile cargo".
Experts say people smuggling is a serious issue in southern Texas, and there have been a number of similar cases in the area just in this past month.
On 7 July, US Border Patrol agents found 72 undocumented immigrants from Central American countries locked inside a trailer "with no means of escape".
The next day 33 people were found locked inside a trailer at a checkpoint on the road to San Antonio.
A truck driver has been charged with illegally transporting immigrants after dozens of people were discovered in the back of his trailer.