SAN ANTONIO (CBS News) — At least 50 migrants were found dead in the back of a tractor-trailer in San Antonio on Monday, federal law enforcement officials confirmed to CBS News on Tuesday. Sixteen others — 12 adults and four children — were taken to local hospitals with heat-related injuries.

The federal officials confirmed to CBS News Homeland Security and Justice reporter Nicole Sganga that this appears to be the deadliest human smuggling case in modern U.S. history.

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On Tuesday morning, Mexican Foreign Relations Secretary Marcelo Ebrard said on Twitter, citing U.S. authorities, that the victims included 22 Mexicans, 7 Guatemalans, and two Hondurans. Investigators are still determining the nationalities of those who died or were hospitalized, Sganga reports.

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Police Chief William McManus said at a press conference Monday that authorities received a call at approximately 5:50 p.m. local time from a worker who had heard cries for help and found the trailer with its doors partially open. The worker found a number of dead bodies inside the trailer, McManus said.

McManus said three people were in custody, though he noted that authorities aren’t certain they’re connected to the situation. The incident is now under federal investigation.

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