Eight suspected illegal immigrants have been found dead inside a truck parked behind a Walmart store in Texas, authorities say.
A ninth victim died later in hospital after being rescued alongside around 30 others. Many were suffering from heat stroke and exhaustion in the trailer, which lacked air conditioning or a water supply, San Antonio Fire Chief Charles Hood said.
The discoveries were made after a Walmart employee was approached for water by one of the travellers who had managed to escape from the vehicle in the early hours of Sunday.
Twenty people were airlifted to seven hospitals and their conditions were "critical to very critical", Mr Hood said. Eight others were admitted to hospital in a less serious condition. The people range from school-age children to adults in their twenties and thirties, he said.
San Antonio Police Chief William McManus said many of the those who survived would suffer severe permanent brain damage.
Firefighters and paramedics arrived on the scene shortly after midnight, Mr Hood said. Officials determined that the truck had reached over 100F (37.7C).
The temperature in San Antonio reached 101F (38C) on Saturday and didn't dip below 90F (32C) until after 10 pm.
“[The passengers] were very hot to the touch,” Mr Hood said. “Each one of them had heart rates over about 130 beats per minute.”
Some of the travellers scattered into the surrounding forest when authorities arrived, according to reports. Officials are now trying to identify the dead and locate those who fled. San Antonio police said the case has been transferred to federal Immigrations and Customs Enforcement (ICE).
Thomas Homan, ICE acting director, condemned the “horrific crime” in a statement on Sunday morning.
“By any standard, the horrific crime uncovered last night ranks as a stark reminder of why human smuggling networks must be pursued, caught and punished," he said. "These networks have repeatedly shown a reckless disregard for those they smuggle."
In a statement released by ICE, US Attorney Richard Durbin vowed to identify those responsible.
“These people were helpless in the hands of their transporters,” he said. “Imagine their suffering, trapped in a stifling trailer in 100-plus degree heat. The driver is in custody and will be charged.”
Authorities declined to identify the driver, who was driving a Pyle Transportation truck. Brian Pyle identified himself to The Washington Post as the owner of the company, but said the driver operated largely independently.
“[The driver] had my name on the side and I pay for his insurance. He makes his own decisions, buys his own fuel,” Mr Pyle said, adding that he had no idea what the man was transporting.
Authorities say the driver could face federal and state charges.
"[This] is not an isolated incident,” the San Antonio police chief warned. “This happens quite frequently ... Fortunately there are people who survived, but this happens all the time.”
This is not the first time suspected illegal immigrants have died while being transported into the country. In one of the deadliest cases on record in the US, 19 immigrants locked inside a truck died in Victoria, Texas, in 2003.
President Donald Trump has vowed to make human trafficking a priority in federal immigration enforcement, claiming the problem is “not talked about enough”. His daughter, Ivanka Trump, has championed the cause, leading a roundtable discussion on the issue at the White House and helping unveil an international human trafficking report.
But immigrants’ rights advocates say Mr Trump’s crackdown on immigration will only make the problem worse, forcing victims of human trafficking to stay silent for fear of being deported.