(NewsNation) — At least 51 migrants have died, 39 men and 12 women, in what appears to be one of the deadliest migrant-smuggling incidents in U.S. history.

On Monday, authorities found dozens of migrants abandoned in the back of a sweltering tractor-trailer near San Antonio, Texas after a city worker heard a cry for help and discovered the gruesome scene.

An initial investigation by law enforcement suggests this week’s tragedy was likely the result of human smuggling — a crime that’s become a daily occurrence along the southern border.

Earlier this week, Border Patrol agents in Del Rio found 43 migrants hiding inside a train car. Even at night, local temperatures stayed above 90 degrees, which means Border Patrol’s intervention likely saved the lives of those in the railcar.

The latest tragedy comes as migrant encounters and deaths along the southern border continue to rise.

At least 650 migrants died crossing the U.S.-Mexico border in 2021, the highest number since 2014 when the International Organization for Migration, a part of the United Nations, began collecting the data.

Border experts say drug cartels are profiting off the current crisis.

“These criminal organizations, they just don’t care about human life,” said Brandon Judd, president of the National Border Patrol Council, the labor union that represents Border Patrol agents.

Judd says human smuggling has become the cartel’s biggest cash cow — in some cases, criminal organizations make tens of thousands of dollars for each migrant smuggled.

Last month, NewsNation joined the Cochise County Sheriff’s Office for an exclusive ride-along and saw firsthand the devastating toll human smuggling takes on migrants and American citizens alike.

Over the course of a few hours, the sheriffs in southeastern Arizona apprehended four migrants. In that specific case, the smugglers were U.S. citizens from Phoenix who picked up the migrants after they crossed the border illegally. The driver was caught after being pulled over for a minor traffic violation.

“By the end of tonight’s shift at 10 p.m. or 11 p.m,. we’ll see about 10 of these types of vehicles,” Cochise County Deputy Bobby Zavala told NewsNation.

The cartels are even active on social media and use TikTok videos to recruit American drivers, many of whom are younger than 18.

But even amid the recent uptick in border encounters, authorities say the scale of the tragedy in Texas is uncommon and shocking.

“We’re not supposed to open up a truck and see stacks of bodies in there. None of us come to work imagining that,” said San Antonio Fire Department Chief Charles Hood.

In addition to the fatalities, 16 people were hospitalized, including four children, according to law enforcement officials.

“The patients that we saw were hot to the touch, they were suffering from heat stroke, heat exhaustion — no signs of water in the vehicle — it was a refrigerated tractor-trailer but there was no visible working AC unit on that rig,” Hood said.

It remains unclear how dozens of migrants ended up in an abandoned trailer near San Antonio. Law enforcement officials said it would be unusual for a truck filled with migrants to make it through an official port of entry without being detected.

It’s more likely the migrants crossed the border themselves and entered the trailer on U.S. soil.