Authorities now confirm at least nine people are dead after being found trapped inside an overheated tractor-trailer in a Walmart parking Sunday morning,  US Immigration and Customs Enforcement (ICE) said. 

Eight bodies were discovered inside the sweltering semi-truck in San Antonio, Texas and the death toll increased to at least nine after another victim died at a hospital late Sunday, they said. 

Police officers work on a crime scene after eight people believed to be illegal immigrants being smuggled into the United States were found dead inside a sweltering 18-wheeler trailer parked behind a Walmart store in San Antonio, Texas, U.S. July 23, 2017. /VCG Photo

Federal officials said in a statement that 39 people had been in the trailer. San Antonio Police Chief William McManus and Fire Chief Charles Hood told reporters that 30 were taken to hospitals; about 20 were in “extremely severe” or critical condition.

They described the incident as a "human trafficking crime."

The discovery was made after a Walmart employee was approached by a man in the truck, asking for water. 

The truck driver has been arrested while an investigation is ongoing to confirm where the trailer came from and how long had it been parked. 

San Antonio lies a few hours drive from the border with Mexico. 

ICE believed over 100 undocumented immigrants has been sandwiched in the back of the truck during the journey. 

The case will be handed to ICE once the victims undergo treatment, police said.