Fifty people have died after being abandoned in a lorry trailer in the sweltering Texas heat, one of the worst tragedies to claim the lives of migrants smuggled across the border from Mexico to the US.
More than a dozen people were taken to hospital, including four children.
A city worker heard a cry for help from the truck on a lonely San Antonio backroad shortly before 6pm on Monday local time (midnight BST) and discovered the gruesome scene, police chief William McManus said.
Hours later, body bags lay spread on the ground near the trailer and bodies remained inside as authorities responded to the tragedy.
Forty-six people were found dead near the scene, authorities said, and four more people later died in hospital, said Patti Tanner, a spokesperson at Baptist Health System in San Antonio.
Among the dead were 39 males and 11 females, authorities said.
San Antonio mayor Ron Nirenberg said those who died had “families who were likely trying to find a better life”.
“This is nothing short of a horrific human tragedy,” Mr Nirenberg said.
It is among the deadliest of the tragedies that have claimed thousands of lives in recent decades as people attempt to cross the US border from Mexico.
Ten migrants died in 2017 after being trapped inside a truck parked at a Walmart in San Antonio. In 2003, the bodies of 19 migrants were found in a sweltering truck south-east of San Antonio.
White House press secretary Karine Jean-Pierre told reporters aboard Air Force One that President Joe Biden was “closely monitoring the absolutely horrific and heartbreaking reports” from San Antonio.
Ms Jean-Pierre pushed back against some Republican legislators who blamed the administration for the deaths.
“Our prayers are with those who tragically lost their lives, their loved ones as well as those still fighting for their lives. We’re also grateful for the swift work of federal, state and local first responders,” Ms Jean-Pierre said Tuesday.
The home countries of all of the migrants and how long they were abandoned on the side of the road were not immediately known.
Among them, 22 were from Mexico, seven from Guatemala and two from Honduras, Roberto Velasco Alvarez, head of the North America department in Mexico’s foreign relations department, said on Twitter.
“Our condolences,” he tweeted. “All responsible will be brought to justice.”
South Texas has long been the busiest area for illegal border crossings. Migrants ride in vehicles through Border Patrol checkpoints to San Antonio, the closest major city, from which point they disperse across the United States.
Bexar County Judge Nelson Wolff, the county’s top elected official, said authorities believe the truck appeared to come from Laredo, a border city that is more than 150 miles south.
“They had just parked it on the side of the road,” Judge Wolff said. “Apparently had mechanical problems and left it there. The sheriff thinks it came across from Laredo.”
Officials were trying to enlist help from neighbouring counties to help with the number of bodies, he said.
Other incidents have occurred long before migrants reached the US border. In December, more than 50 died when a semitrailer filled with migrants rolled over on a highway in southern Mexico.
In October, Mexican authorities reported finding 652 migrants packed into six trailers near the US border. They were stopped at a military checkpoint.
Officers arrived to find a body on the ground outside the trailer and a partially opened gate to the trailer.
Three people were taken into custody, but it was unclear if they were definitively connected with human trafficking, Mr McManus said. The trailer was gone on Tuesday morning, but access to the area where it was found remained blocked.
The truck was abandoned on the outskirts of San Antonio, next to a railroad track in an area surrounded by auto scrapyards that brush up against a busy freeway.
Of the 16 taken to hospitals with heat-related illnesses, 12 were adults and four were children, said Fire Chief Charles Hood. The patients were hot to the touch and dehydrated, and no water was found in the trailer, he said.