US authorities opened a criminal investigation Tuesday after 50 migrants packed into a stifling trailer died in Texas, with President Joe Biden blaming professional smugglers for the tragedy.

A spokesperson for US Immigration and Customs Enforcement said the toll had risen overnight after the tractor-trailer rig was discovered Monday abandoned on an isolated road near the city of San Antonio.

Sixteen people, including four children, were taken to hospital suffering from extreme dehydration. Most of the fatalities were Mexican, and US media said some victims died in hospital.

Biden said the grim incident underscored the need to shut down "the multi-billion dollar criminal smuggling industry preying on migrants."

"The tragic loss of life in San Antonio, Texas that took place yesterday is horrifying and heartbreaking," he said in a statement.

- Intense heat -

It was the most deadly single incident involving migrants along the southern border in memory, and drew more attention to the risks that hundreds of thousands of people face seeking to enter the United States from Mexico without permission.

RTL

The intense heat of the region is one of the greatest threats, to those smuggled in vehicles and others who cross on foot and try to make their way into the country across sweltering desert.

On Monday, the high temperature in San Antonio was 103 degrees Fahrenheit, or 39.4 degrees Celsius, and the temperature in the unvented trailer would have been much higher.

According to Mexican President Andres Manuel Lopez Obrador, most of the dead were from Mexico and Central America.

"It's a tremendous misfortune... so far there are 50 dead: 22 from Mexico, seven from Guatemala, two from Honduras and 19 still without information about their nationality," Lopez Obrador told reporters.

- Common route -

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San Antonio Police Chief William McManus said authorities were first alerted to the trailer by an emergency call at about 5:50 pm local time (2250 GMT).

"A worker who works in one of the buildings up here behind me heard a cry for help," he told reporters.

The worker "came out to investigate, found a trailer with the doors partially open, opened them up to take a look, and found a number of deceased individuals inside," McManus said.

San Antonio Fire Chief Charles Hood said it was a refrigerated tractor-trailer but there appeared to be no working air-conditioning.

Three people have been taken into custody but officials gave no further details.

The tragedy came five years after 10 migrants were found dead in a trailer with broken air conditioning and clogged ventilation holes near San Antonio.

In recent weeks Border Patrol officers have discovered other attempts to bring undocumented travelers into the country in large trucks.

On June 14, 80 people from Mexico, Guatemala, Honduras, and El Salvador were discovered inside a tractor-trailer when it was inspected by agents at a highway checkpoint north of Laredo, a border hub in southeastern Texas.

Three weeks earlier, agents intercepted a trailer with 48 people inside near Sierra Blanca in western Texas.

- 'Desperation of migrants' -

San Antonio, which lies 250 kilometers (150 miles) from the border, is a major transit route for human smugglers as well as tens of thousands of migrants who cross the border, request asylum, and are allowed to continue on.

"This speaks to the desperation of migrants who would put their lives in the hands of callous human smugglers who show no regard for human life," said US Customs and Border Protection agency director Chris Magnus.

The case immediately became a focus of politics when Republicans attacked Democratic President Biden for allegedly being soft on immigration.

"These deaths are on Biden. They are a result of his deadly open border policies," said Texas Governor Greg Abbott.

Under Biden more than 200,000 people attempting to enter the country illegally have been interdicted at the border each month and sent back.

But there is no good estimate of the thousands more that succeed in staying inside the country.

Biden said he had already launched an anti-smuggling campaign that focused on the networks and arrested 2,400 people in recent months.

"Exploiting vulnerable individuals for profit is shameful, as is political grandstanding around tragedy," he said.