An American driver has been arrested in Texas after border agents found 53 illegal immigrants from Mexico and several other countries south of the U.S. border hidden inside a sweltering tractor-trailer, authorities said Friday.

The unidentified driver was the only visible occupant inside the vehicle when it pulled up to the Interstate Highway 35 (IH-35) checkpoint north of Laredo on Tuesday, U.S. Customs and Border Protection (CBP) said in a statement.

ARIZONA DRIVER ARRESTED AFTER 31 ILLEGAL IMMIGRANTS FOUND IN TRACTOR-TRAILER, INVESTIGATORS SAY

A canine alerted border agents to the odors of concealed humans or narcotics from inside the trailer during a primary inspection of the vehicle, the CPB said. Upon further examination, agents said they discovered 53 migrants from Mexico, Ecuador, Guatemala, El Salvador and Honduras, who were "all illegally present in the United States."

The temperature inside the trailer was recorded at 105 degrees at the time of the discovery, though no one required medical attention, according to the CPB.

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Agents arrested the U.S. citizen and all 53 illegal immigrants, who they said would be "processed accordingly." Border Patrol seized the tractor-trailer.

It was unclear whether the driver is facing charges, but investigators called the incident a "human smuggling attempt."

Homeland Security Investigations is handling the case.