Fifty people have died after being abandoned in a tractor-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 U.S. More than a dozen people had been taken to hospitals, including four children.

A city worker heard a cry for help from the truck on a lonely San Antonio back road shortly before 6 p.m. Monday 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 calamity.

Forty-six people were found dead near the scene, authorities said. Four more later died after being taken to hospitals, said Bexar County Judge Nelson Wolff, the county’s top elected official,

Among the dead were 39 males and 11 females, he 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,” Nirenberg said.

It’s among the deadliest of the tragedies that have claimed thousands of lives in recent decades as people attempt to cross the U.S. 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 southeast 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.

Jean-Pierre pushed back against some Republican lawmakers who blamed the administration for the deaths and said it was focused on the victims and holding human smugglers accountable.

“The fact of the matter is, the border is closed, which is in part why you see people trying to make this dangerous journey using smuggling networks,” she said. “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.”

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 Álvarez, 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.

Wolff said Tuesday that authorities believe the truck came from Laredo, a border city that is more than 150 miles (241 kilometer) south.

“They had just parked it on the side of the road,” 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 neighboring counties to help with the number of bodies, he said.

Other incidents have occurred long before migrants reached the U.S. 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 U.S. 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, McManus said. The trailer was gone Tuesday morning, and 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.

Sixteen people were taken hospitals with heat-related illnesses, with four later dying. At least four were in critical condition, according to the hospitals. Among those were a 23-year-old woman in serious condition and an adolescent male in critical condition at University Hospital in San Antonio, Wolff said.

Those taken to the hospital were hot to the touch and dehydrated, and no water was found in the trailer, said Fire Chief Charles Hood.

“They were suffering from heat stroke and exhaustion,” Hood said. “It was a refrigerated tractor-trailer, but there was no visible working AC unit on that rig.”

Those in the trailer were part of a presumed migrant smuggling attempt into the United States, and the investigation was being led by U.S. Homeland Security Investigations, McManus said.

Big rigs emerged as a popular smuggling method in the early 1990s amid a surge in U.S. border enforcement in San Diego and El Paso, Texas, which were then the busiest corridors for illegal crossings.

Before that, people paid small fees to mom-and-pop operators to get them across a largely unguarded border. As crossing became exponentially more difficult after the 2001 terror attacks in the U.S., migrants were led through more perilous terrain and paid thousands of dollars more.

Heat poses a serious danger, particularly when temperatures can rise severely inside vehicles. Weather in the San Antonio area was mostly cloudy Monday, but temperatures approached 100 degrees Fahrenheit (38 degrees Celsius).

Some advocates drew a link to the Biden administration’s border policies. Aaron Reichlin-Melnick, policy director at the American Immigration Council, wrote that he had been dreading such a tragedy for months.

“With the border shut as tightly as it is today for migrants from Mexico, Guatemala, Honduras and El Salvador, people have been pushed into more and more dangerous routes. Truck smuggling is a way up,” he wrote on Twitter.

Migrants — largely from Mexico, Guatemala, Honduras and El Salvador — have been expelled more than 2 million times under a pandemic-era rule in effect since March 2020 that denies them a chance to seek asylum but encourages repeat attempts because there are no legal consequences for getting caught. People from other countries, notably Cuba, Nicaragua and Colombia, are subject to Title 42 authority less frequently due to higher costs of sending them home, strained diplomatic relations and other considerations.

U.S. Customs and Border Protection reported 557 deaths on the southwest border in the 12-month period ending Sept. 30, more than double the 247 deaths reported in the previous year and the highest since it began keeping track in 1998. Most are related to heat exposure.

CBP has not published a death tally for this year but said that the Border Patrol performed 14,278 “search-and-rescue missions” in a seven-month period through May, exceeding the 12,833 missions performed during the previous 12-month period and up from 5,071 the year before.