A number that has risen since initial reports, 50 migrants, most of them from Mexico, were found dead in the back of a tractor trailer parked in San Antonio, Texas.

Described as a “horrific human tragedy” by the mayor, there were a total of 16 survivors, four of them children, while four others died at the hospital. Three people are now in police custody in relation to the mass casualty.

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San Antonio Fire Chief Charles Hood said the bodies were “hot to the touch,” per a statement made on Monday. “They were suffering from heat stroke, heat exhaustion. No signs of water in the vehicle. It was a refrigerated tractor-trailer, but there was no visible working AC unit on that rig.”

Temperatures in Texas on Monday reached 99 degrees.

Law enforcement say they were alerted by a city worker who heard a cry for help at around 6 p.m. near Interstate 35, a major highway leading to the U.S.-Mexico border.

They arrived at the grisly scene to find a body outside of the trailer and a trailer gate that had been partially opened. By Tuesday morning, the trailer had been cleared from the scene, but the road on which it was found was still cordoned off.

Among the victims identified, 22 were from Mexico, seven were from Guatemala and two were from Honduras.

Overall, 2022 is gearing up to be one of the biggest years on record for migration, and experts are anticipating 2 million total arrests by the end of the year. There were 1.7 million migrant arrests in 2021, a record number.

This rapid influx of migrants to the United States has also meant a higher rate of migrant mortality, especially with record-breaking heat making the journey all the more difficult.

Reports say that U.S. Customs and Border Protection made almost 240,000 arrests in May alone, as temperatures across Texas have approached record highs. The day before authorities discovered the bodies, temperatures in San Antonio had reached 103 degrees, one degree short of the city’s record high of 104 in 2012.

The Biden administration faced criticism from Republican lawmakers, most notably Texas Gov. Greg Abbott’s criticizing Biden’s “deadly open border policies.” White House press secretary Karine Jean-Pierre said, “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,” according to AP.

It’s unclear whether the truck came from across the border or whether it had been loaded with migrants domestically. After successfully crossing over, many migrants are packed into tractor-trailers like the one to evade CBP checkpoints along the major highways.

Bexar County Judge Nelson Wolff said that local law enforcement thinks the truck came from a nearby town called Laredo, which rests on the border 150 miles south of Bexar County. “They had just parked it on the side of the road,” he said. “Apparently had mechanical problems and left it there. The sheriff thinks it came across from Laredo.”

The investigation is now being led by Homeland Security investigators, in collaboration with CBP, local law enforcement and the U.S. Immigrations and Customs Enforcement.

According to the New York Post, CBP has performed more than 14,000 “search-and-rescue missions” in the seven-month period that ended this past May. The entire 12-month period prior saw 13,000 missions, with just over 5,000 in the year prior.