WASHINGTON - The attempt to smuggle more than 70 people from Mexico into Texas this week was bold: A driver pulled a tractor-trailer into the commercial vehicle inspection lanes at a U.S. Border Patrol checkpoint in Laredo, and waited for agents to check the truck.

From a photo released by U.S. Customs and Border Protection, there was little else in the trailer except for dozens of people huddled against a metal-clad wall, heads turned to avoid the camera or the bright light shined at them.

Most of those visible in the picture were wearing white shirts, which CBP says were marked by the smugglers "to assist the trafficking organization in classifying/identifying the individuals within the group."

It is unclear what the markings were, and what they signified. A request to CBP for more information was not immediately returned on Wednesday. The story will be updated with the agency's response.

The group was comprised of men and women from El Salvador, Guatemala and Mexico who entered the United States without authorization, according to CBP.

The driver, a U.S. citizen, was arrested, as were the people found in the trailer.

While many migrants attempt to cross the border into the U.S. on foot to remain undetected, that often requires swimming across the river or traversing remote, rugged desert areas where deaths by drowning and dehydration regularly occur.

2019 Among Deadliest Years for Migrants Trekking to US By the end of August, more than 520 migrants in the Americas died or went missing and are presumed to have perished

The International Organization for Migration, which tracks migrant deaths around the world, reports that as of Dec. 16, 351 people have died along the U.S.-Mexico border this year.

Some people attempt to cross hidden in car trunks or truck beds. Others pay to be transported in large tractor-trailers, where they are sometimes concealed by cargo.

US Border Agents Find Dozens of Migrants Inside Produce Truck Two Americans are facing human smuggling charges after 32 Mexican and Ecuadorian nationals were found hidden inside a chilled semitrailer

Although vehicles can provide protection from poor weather conditions, they leave large groups of migrants vulnerable to the network of people involved in smuggling, including their drivers.

In 2018, a truck driver was sentenced to life in prison after a botched smuggling attempt killed 10 people in San Antonio, Texas.

CBP announced last week that apprehensions at the southwest U.S. border decreased for a sixth month, after spiking from late 2018 to May of this year.