Photo taken on June 27, 2022 shows the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio, a major city in south central U.S. state Texas, has risen to 50, authorities said on Tuesday.

The tragedy appeared to be one of the deadliest migrant-smuggling operations in recent U.S. history, local media reported.

Forty-six people were found dead at the scene and four others were later pronounced dead at area hospitals, Texas Public Radio reported, citing Mexican Foreign Minister Marcelo Ebrard.

Ebrard said 22 were Mexican nationals, seven were from Guatemala, two were from Honduras, and the remaining 19 dead have not yet been identified.

At least 16 others, including four children, were taken to area hospitals due to heat exhaustion and dehydration, local officials said at a news conference. Nearly 100 people were reportedly packed inside the trailer of the 18-wheeler.

The San Antonio Express-News reported that someone "cloned" the truck carrying the migrants with the same color and identifying numbers from the U.S. Department of Transportation and the Texas DOT as a truck owned by a South Texas trucking company, in an apparent move to evade authorities.

Three people are in police custody, but it's unclear if they are involved in the tragedy, San Antonio Police Chief William McManus told a briefing Monday night.

Police received a call on Monday evening from a person working nearby, who heard a cry for help from the 18-wheeler in the southwest of the city, approached and saw bodies inside the trailer with its doors partially open, said McManus.

Those who were found alive were hot to the touch and suffering from heat stroke and exhaustion, and there was no sign of water in the refrigerated tractor-trailer and no visible working air conditioning unit, according to San Antonio Fire Chief Charles Hood.

Temperatures in San Antonio climbed to 101 degrees Fahrenheit (38.3 degrees centigrade) on Monday, while the heat in a packed trailer was likely way higher than that, according to the U.S. National Weather Service.

"Migrants seeking asylum should always be treated as a humanitarian crisis, but this evening we're facing a horrific human tragedy," San Antonio Mayor Ron Nirenberg tweeted on Monday.

Texas Republican Governor Greg Abbott, who is seeking a second term, on Monday night slammed U.S. President Joe Biden for the tragedy, tweeting: "These deaths are on Biden. They are a result of his deadly open border policies."

In July 2017, 10 migrants died in a packed truck carrying 39 people in San Antonio.

Photo taken on June 28, 2022 shows road signs near the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

People gather at the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States, on June 27, 2022. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

Police officers guard the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States, on June 28, 2022. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

First responders work at the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States, on June 27, 2022. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

A police officer stands guard around the perimeter of the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States, on June 27, 2022. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

Police cordon off the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States, on June 28, 2022. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

An ambulance leaves the scene of an alleged human smuggling mass casualty event in San Antonio, Texas, the United States, on June 27, 2022. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)

The scene of an alleged human smuggling mass casualty event is cordoned off in San Antonio, Texas, the United States, on June 28, 2022. The death toll of migrants found on Monday inside a "cloned" and abandoned 18-wheeler in San Antonio has risen to 50, authorities said on Tuesday. (Photo by Nick Wagner/Xinhua)