UPDATE: 11:35 p.m. EDT, Sunday — One of the 30 people earlier found alive in a tractor-trailer parked behind a Walmart store, San Antonio, Texas died while he was being transported to hospital, reports said. The death toll in the immigrant-smuggling attempt has increased to nine after eight likely undocumented migrants were found dead inside the truck.

UPDATE: 6:33 a.m. EDT — The San Antonio Police Department (SAPD) said that they will "look for additional people" the first thing in the morning following the discovery of a semi-trailer at a Walmart car park in San Antonio, Texas, on Sunday morning where eight people were found dead and around 30 others in critical state.

Original Story:

Eight people were dead and around 30 others are in critical state after being found locked in a semi-trailer at a Walmart car park in San Antonio, Texas, on Sunday morning, the police said. They are still investigating the case.

The truck was apparently involved in smuggling-related operations with undocumented immigrants, authorities pointed it out.

They also said at least 29 fire units and two AirLife choppers responded to the scene to take care of the victims, according to KENS5.

Kurt Chirbas from NBC News tweeted 17 were in critical condition and 13 others were in non-life threatening situation.

Social media users have been regularly updating about the incident. Joseph Lopez from KENS5, a CBS-affiliated TV station located in San Antonio, tweeted that the incident is likely to be a "human trafficking crime" as told by the chief of San Antonio Police Department (SAPD). It still remains unclear where the truck came from. 

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Among the eight dead were two young children. The injured people have been transported to hospitals with possibly life-threatening injuries, according to KENS5.

The Walmart where the incident took place, is located at I-35 and Highway 16. Firefighters were called to the scene around 12:30 a.m. local time (1:30 a.m. EDT) and found the dead people inside the wheeler.

At a press conference, Police Chief William McManus said the driver of the truck, who hasn't been identified yet, is in custody. He added the driver is likely to face both state and federal charges, WCVB5 reported.

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

The incident came to light when the police got a call from a Walmart employee about a welfare check in a tractor-trailer that was parked on the lot. The employee 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,” the police chief said, adding that it was a “horrific tragedy.”

Officials said this was not a standalone incident as it happens frequently but they were fortunate to find out about this tragedy. 

The Department of Homeland Security is assisting in the investigation.

Read: Forced Labor Comes In Unexpected Forms In Human Trafficking, Study Says 

The Statewide Human Trafficking Mapping Project of Texas, launched in 2014 with support from the Criminal Justice Division of the Office of the Texas Governor, found in study published in 2016 that there are more than 300,000 victims of human trafficking in Texas, which includes 79,000 minors and youth victims of sex trafficking. There are about 234,000 adult victims of labor trafficking.

Commercial sex, forced labor, domestic servitude are the purposes for which human trafficking takes place. A human trafficking victim is controlled through violence, deception or coercion in these circumstances.