WASHINGTON - Two people face human smuggling charges after U.S. border agents found dozens of Mexican and Ecuadorian nationals hidden inside a chilled semitrailer in the U.S. state of Arizona this week.

The 31-year-old driver and a 30-year-old passenger, both U.S. citizens not named by border officials, were stopped at an immigration checkpoint Monday on Interstate 19, a highway that runs from the U.S.-Mexico border, through the city of Tucson.

After a working dog alerted agents, a heat scan showed that inside the 8-degree-Celsius refrigerated tractor-trailer were 32 people hiding among produce, according to a U.S. Customs and Border Protection statement issued Wednesday.

Ranging in age from 16 to 53, the migrants entered the U.S. without authorization and were processed for immigration violations, CBP officials said.

Photos tweeted by the White House show a group of men sitting in the trailer with their hands up, some covering their faces, as a light is shone at them.

Informal smuggling networks along the southwest U.S. border regularly use trucks, cars, vans, and buses to transport people without authorization into the United States.

While trucks can be an appealing option, with protection from extreme temperatures and faster transportation, they also put travelers at risk, dependent on drivers and others involved in the operation for their safety.

Last year, a truck driver was sentenced to life in prison for his role in a smuggling operation that left 10 people dead in San Antonio, Texas.