SAN ANTONIO (AP) — Desperate families of migrants from Mexico and Central America frantically sought word of their loved ones as authorities began the grim task Tuesday of identifying 51 people who died after being abandoned in a tractor-trailer without air conditioning in the sweltering Texas heat.

It was the deadliest tragedy to claim the lives of migrants smuggled across the border from Mexico.

The driver of the truck and two other people were arrested, U.S. Rep. Henry Cuellar of Texas told The Associated Press.

He said the truck had passed through a Border Patrol checkpoint northeast of Laredo, Texas, on Interstate 35. He did not know if migrants were inside the truck when it cleared the checkpoint.

Investigators traced the truck’s registration to a residence in San Antonio and detained two men from Mexico for possession of weapons, according to criminal complaints filed by the U.S. attorney’s office. The complaints did not make any specific allegations related to the deaths.

The bodies were discovered Monday afternoon on the outskirts of San Antonio when a city worker heard a cry for help from the truck parked on a lonely back road and found the gruesome scene inside, police Chief William McManus said. Hours later, body bags lay spread on the ground.

More than a dozen people — their bodies hot to the touch — were taken to hospitals, including four children. Most of the dead were males, he said.

The death count was the highest ever from a smuggling attempt in the United States, according to Craig Larrabee, acting special agent in charge of Homeland Security Investigations in San Antonio.

“This is a horror that surpasses anything we’ve experienced before,” San Antonio Mayor Ron Nirenberg said. “And it’s sadly a preventable tragedy.”

President Joe Biden called the deaths “horrifying and heartbreaking.”

“Exploiting vulnerable individuals for profit is shameful, as is political grandstanding around tragedy, and my administration will continue to do everything possible to stop human smugglers and traffickers from taking advantage of people who are seeking to enter the United States between ports of entry,” Biden said in a statement.

Authorities did not know the home countries of all of the migrants, nor how long they were abandoned on the side of the road.

By Tuesday afternoon, medical examiners had potentially identified 34 of the victims, but they were taking additional steps, such as fingerprints, to confirm the identities, said Bexar County Commissioner Rebeca Clay-Flores.

Among the dead, 27 are believed to be of Mexican origin based on documents they were carrying, according to said Rubén Minutti, Mexico consul general in San Antonio. Several survivors were in critical condition with injuries such as brain damage and internal bleeding, he said.

At least seven of the dead were from Guatemala and two from Honduras, Roberto Velasco Álvarez, head of the North America department in Mexico’s Foreign Relations Department, said on Twitter. About 30 people had reached out to the Mexican Consulate looking for loved ones, the officials said.

Authorities confirmed that one of the surviving Mexicans from the trailer was José Luis Guzmán Vásquez, 32, from San Miguel Huautla in the southern state of Oaxaca, according to Aida Ruiz García, director of the Oaxacan Institute for Migrant Attention. He was dehydrated and receiving care at a San Antonio hospital, Mexico’s foreign affairs said.

A cousin, Alejandro López, told Milenio television that the family worked in farming and construction and migrated because “we don’t have anything but weaving hats, palms and handicrafts.”

Attempts to cross the U.S. border from Mexico have claimed thousands of lives in both countries in recent decades.

U.S. border authorities are stopping migrants more often on the southern border than at any time in at least two decades. Migrants were stopped nearly 240,000 times in May, up by one-third from a year ago.

Comparisons to pre-pandemic levels are complicated because migrants expelled under a public health authority known as Title 42 face no legal consequences, encouraging repeat attempts. Authorities say 25% of encounters in May were with people who had been stopped at least once in the previous year.

South Texas has long been the busiest area for illegal border crossings. U.S. authorities discover trucks with migrants inside “pretty close” to daily, Larrabee said.

Migrants typically pay $8,000 to $10,000 to be taken across the border and loaded into a tractor-trailer and driven to San Antonio, where they transfer to smaller vehicles for their final destinations across the United States, he said.

Conditions vary widely, including how much water passengers get and whether they are allowed to carry cellphones, Larrabee said.

Authorities think the truck discovered Monday had mechanical problems when it was left next to a railroad track in an area of San Antonio surrounded by auto scrapyards that brush up against a busy freeway, said Bexar County Judge Nelson Wolff, the top elected official in the county that includes San Antonio.

San Antonio has been a recurring scene of tragedy and desperation in recent years involving migrants in semitrailers.

Ten migrants died in 2017 after being trapped inside a truck parked at a San Antonio Walmart. In 2003, the bodies of 19 migrants were found in a sweltering truck southeast of the city. More than 50 migrants were found alive in a trailer in 2018, driven by a man who said he was to be paid $3,000 and was sentenced to more than five years in prison.

Other tragedies have occurred long before migrants reached the U.S. In December, more than 50 died when a semitrailer rolled over on a highway in southern Mexico. In October, Mexican authorities reported finding 652 migrants packed into six trailers stopped at a military checkpoint near the border.

Some of the 16 people taken to hospitals with heat-related illnesses remained hospitalized Tuesday in critical condition.

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.”

Temperatures in San Antonio on Monday approached 100 degrees Fahrenheit (38 degrees Celsius).

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.

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

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,” he wrote on Twitter.

During a vigil held Tuesday evening in the rain at a San Antonio park, many of the more than 50 people who attended expressed sadness, frustration and anger at the deaths and what they described as a broken immigration system.

“I see this happen, and it didn’t have to happen. If we had a better way for brown and Black people to enter safely, they wouldn’t go through these desperate measures,” said San Antonio resident Debbie Ponce.

Migrants — largely from Mexico, Guatemala, Honduras and El Salvador — have been expelled more than 2 million times under the pandemic-era rule in effect since March 2020 that denies a chance to seek asylum. The Biden administration planned to end the policy, but a federal judge in Louisiana blocked the move in May.

U.S. Customs and Border Protection reported 557 deaths on the Southwest border in the 12 months 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 were related to heat exposure.

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Spagat reported from San Diego. Associated Press reporters Eric Gay in San Antonio, Acacia Coronado in Austin, Terry Wallace in Dallas and Ken Miller in Oklahoma City contributed.