An unidentified driver was arrested at a checkpoint in Texas after border agents found 53 illegal immigrants from Mexico and several other countries south of the U.S. border hidden in the back of a sweltering tractor-trailer, the U.S. Border Patrol said on Friday, Sept. 20.

The migrants were found Tuesday, Sept.17, during a search of the truck at a checkpoint on Interstate 35 north of Laredo. They are from the countries of Mexico, Ecuador, Guatemala, El Salvador, and Honduras, all illegally present in the United States, and the driver a U.S. citizen, were placed under arrest and processed accordingly, U.S. Customs and Border Protection (CBP) said in a statement.

The temperature inside the trailer was 105 degrees Fahrenheit at the time of the discovery, no onerequired medical attention, according to the CPB.

The tractor-trailer was seized by the U.S. Border Patrol.

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

Homeland Security is investigating the case.