An Arizona man has been arrested and accused of human smuggling after Border Patrol agents found 31 illegal immigrants from Mexico in the tractor-trailer he was driving, U.S. Customs and Border Protection announced Tuesday.

Agents assigned to the Interstate 19 immigration checkpoint near Amado made the discovery after a Border Patrol dog alerted them to the tractor-trailer for a secondary inspection last week, officials said.

That is when agents discovered the immigrants, who were between the ages of 15 and 40, sitting inside the cargo trailer.

A scan of the tractor-trailer holding illegal immigrants. The driver was arrested. (U.S. Customs and Border Protection)

The 31 Mexican nationals, who were “all illegally present in the country,” were arrested as well, investigators said, adding that they will be processed for immigration violations.

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Agents assigned to the I-19 immigration checkpoint discovered 31 Mexican nationals inside the back of the tractor-trailer. (U.S. Customs and Border Protection)

They also said the 40-year-old driver, who has not yet been identified, was an American living in Nogales, Ariz.

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Border Patrol officials said the Department of Homeland Security launched "Operation Safeguard" to educate the trucking industry and the public on the life-threatening dangers involved in using commercial vehicles to smuggle people.

The operation was created in an effort to prevent deaths at the hands of smugglers.