A U.S. official says at least 46 people are dead after bodies were found inside a tractor-trailer in a presumed migrant smuggling attempt in South Texas.

The official says 16 others in the truck were taken to hospitals in San Antonio, where the bodies were found Monday.

The official initially spoke to The Associated Press on condition of anonymity because the information had not been authorized for public release.

During a news conference, San Antonio officials confirmed the incident, calling it a "mass casualty event" that is now at the center of a federal investigation.

Police responded to a call late Monday and found a trailer with its doors partially open. Upon their investigation, authorities quickly realized there were "a number of deceased people inside."

Marcelino Benito with KHOU was on the scene during the press conference where officials described the horrific scene.

While authorities gave numbers on both the deceased and the survivors, police may have reason to believe there are more victims that haven't been found yet.

Currently, three people are in custody although police would not reveal how or why they are believed to be connected to this tragic event.

San Antonio city leaders described the incident as a "horrific human tragedy."

This is a developing story and we will continue to update it as more information becomes available.

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