Dozens of migrants were found dead in an abandoned big rig in San Antonio on Monday in what appears to be the deadliest human smuggling case in modern U.S. history.

The bodies of at least 46 people were initially found in the tractor-trailer in the sweltering Texas heat, officials said. Sixteen others, including four children, were hospitalized, San Antonio Fire Chief Charles Hood said.

On Tuesday morning, Mexican President Andrés Manuel López Obrador said the death toll had risen to 50. He said 22 of the dead were Mexican nationals, while seven were from Guatemala and two from Honduras. The nationalities of the remaining 19 people had yet to be confirmed.

López Obrador said the Mexican government would be providing assistance to the family members of the dead.

Three people were taken into custody following the discovery, San Antonio Police Chief William P. McManus said, though he added authorities did not know if they were definitely connected to the incident. He did not expand on their identities.

The grim discovery was made early Monday evening in an undeveloped area of southwest San Antonio near railroad tracks. A person who works in the area reported hearing a cry for help and spotted at least one body, officials said.

“We’re not supposed to open up a truck and see stacks of bodies in there,” Hood said.

McManus said the survivors lacked water and air conditioning. “The patients that we saw were hot to the touch,” he said. “They were suffering from heat stroke, heat exhaustion.”

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