Authorities in Texas are hunting for a human-trafficking gang after eight people were found dead in the back of a tractor-trailer parked at a Walmart lot in San Antonio on early Sunday (23 July).

William McManus, San Antonio Police Chief said they discovered the bodies after a phone call late Saturday.

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"We're looking at a human-trafficking crime here," said McManus. "Homicide will work to determine the origin of this horrific tragedy.

"We got a call from a Walmart employee about a welfare check in a tractor-trailer that was parked on the lot here."

"He was approached by someone from that truck, who was asking for water. Came back with a water, called the police and we arrived on scene and found eight people dead in the back of that trailer," he added.

When the trailer was opened by the police, 38 people were discovered inside the back of the non-air-conditioned vehicle.

According to San Antonio Fire Chief Charles Hood, eight people were pronounced dead at the scene. Some 20 others have been hospitalized in 'extremely critical' or 'very serious' condition.

The remaining 10 people are being treated for less severe injuries. Among the people inside the trailer, two were school-aged children. Police said the rest are in their 20s and 30s.

The driver of the truck, who has not been identified, is in custody and is expected to face both state and federal charges, Chief McManus said.

McManus said that the store surveillance video showed a number of vehicles picking several people up from inside the trailer before authorities could arrive on the scene.

The Department of Homeland Security are assisting in the investigation.