It was on Monday that a total of 67 people were found in a truck at the roadside in San Antonio in southern Texas. At the time of discovery, 46 of them had died in the heat.
About 15 survivors were taken to hospital, but many were very difficult to take and the number of deaths has risen to 53.
Investigators are investigating whether there may have been technical problems that made the truck seemingly abandoned near a railway track. A worker working in one of the nearby buildings heard cries for help, and when the tragedy was discovered, the suspected driver tried to blend in with the migrants, says Francisco Garduño, head of the Mexican Migration Agency.
The suspected driver is a US citizen and is under arrest along with three others for involvement in human trafficking. Two of the suspects, including the driver, risk life imprisonment or the death penalty for the crimes they are suspected of, according to federal prosecutors.
At least 27 of the dead were from Mexico, according to Garduño. Others came from different countries in Central America, but the identification of the victims is still ongoing.
San Antonio is 250 km from the border with Mexico and is one of the main routes for human traffickers. The tragedy is believed to be the deadliest of its kind that has occurred in the United States along the border with Mexico.