Members of law enforcement investigate a tractor-trailer in San Antonio, Texas, on Monday. AFP

The deceased migrants, 39 men and 12 women, most of them citizens of Mexico, were discovered on Monday in an industrial area on the outskirts of San Antonio, Texas, about 160 miles (250km) north of the US-Mexico border.

READ MORE

Fire kills 52 after apparent riot attempt at Colombia prison

Dozens missing after strike on Ukraine mall, Russia presses attacks on east

The incident marked the greatest loss of life on record from a human trafficking attempt in the United States, according to Craig Larrabee, an acting special agent in charge with the investigative arm of US Immigration and Customs Enforcement (ICE).

Local authorities described finding the rear door to the trailer open with "stacks of bodies" inside, while others were strewn collapsed nearby. Some of the victims were hot to the touch, they said.

Police and other first responders work the scene in San Antonio. AP

Two suspects identified as Juan Francisco D'Luna-Bilbao and Juan Claudio D'Luna-Mendez, both Mexican citizens, have been charged with possessing firearms while residing in the United States illegally, according to court documents and US authorities.

Investigators traced the truck's vehicle registration to a San Antonio address that they placed under surveillance, and arrested the two men separately when each was seen leaving the residence.

A third suspect, described as a US citizen who drove the truck, has also been taken into custody and was expected to be charged, but he remained hospitalised as of Tuesday evening, according to a Mexican official.

The truck had been left parked beside some railroad tracks as temperatures soared as high as 103 Fahrenheit (39.4 Celsius). Local authorities said there were no signs of water or visible means of air-conditioning inside the truck.