The tractor-trailer was pitch-black inside, crammed with maybe 90 immigrants or more, and already hot when it left the Texas border town of Laredo for the 150-mile trip north to San Antonio.
The details of the journey were recounted Monday by a survivor who spoke to The Associated Press and in a federal criminal complaint against the driver, James Matthew Bradley, who could face the death penalty over the 10 lives lost.
"After an hour I heard ... people crying and asking for water. I, too, was sweating and people were despairing. That's when I lost consciousness," 27-year-old Adan Lalravegas told the AP from his hospital bed.
In addition to the dead, nearly 20 others rescued from the rig were hospitalized in dire condition, many suffering from extreme dehydration and heatstroke.
A number of those aboard were from Mexico and Guatemala. Many of the immigrants had apparently hired smugglers who brought them across the U.S. border, hid them in safe houses and then put them aboard the tractor-trailer for the ride northward, according to accounts given to investigators.