The unbearable conditions inside semi-trailer where eight people were found dead.

A truck driver has been charged in Texas with illegally transporting dozens of immigrants in his tractor-trailer in sweltering heat that led to the deaths of 10 people.

James Matthew Bradley Jr, 60, was arrested on Sunday after authorities found eight men dead in the back of his truck parked outside a Walmart store in San Antonio along with scores of other immigrants, including children, suffering from dehydration and heat stroke.

Two people died later at hospitals and dozens of others were treated, officials said.

Some immigrants came from Mexico, officials said, and at least two Guatemalans were found alive but dehydrated, the Guatemalan foreign ministry said. Outside temperatures topped 37.8 degrees Celsius on Sunday.

Bradley told investigators at the scene he did not know anyone was inside the truck until he parked near the Walmart to use the bathroom, according to the criminal complaint.

Bradley appeared briefly in court to be charged under a law that makes it illegal to transport an immigrant while knowing the person is in the country illegally or in "reckless disregard" of that fact, according to a complaint filed in federal court in San Antonio, about 150 miles (240 km) north of the US-Mexico border.

If convicted on the single count, Bradley could face the death penalty or life in prison.

San Antonio police found Bradley inside the tractor-trailer when they arrived, according to an account in the criminal complaint.

He told federal investigators he was driving the tractor-trailer from Iowa to Brownsville, Texas, to deliver it to the new owner. After parking outside the Walmart, he heard banging and shaking.

Bradley opened the doors "and was surprised when he was run over by 'Spanish' people and knocked to the ground," according to the complaint's summary of the interview. He noticed "bodies just lying on the floor like meat." Some 30 or 40 people got out and "scattered," Bradley said.

He called his wife but did not call 911 for emergency services, according to the court documents.

The complaint also included summaries of interviews with people inside the truck. One said conditions became so unbearable inside the pitch-black interior that people took turns to breathe through a hole in the vehicle's side. Others passed out.

The immigrants, who were not named in the complaint, all described getting on the truck at various points on the US side of the border. One said 70 people were already inside when he got on, while another estimated the total at 180 to 200 people.

Six black sports utility vehicles were waiting outside the Walmart when the truck doors were opened, one rider said, and some of the immigrants quickly "swarmed" the SUVs. That contradicted the account of Bradley, who told investigators he did not see any vehicles waiting to pick up passengers.

San Antonio Police Chief William McManus said video footage showed several other vehicles coming to pick up people who were inside the truck.