Immediately after President Trump's inauguration in 2017, arrests of illegal immigrants on the southwest border plummeted to lows that hadn't been seen in years. But three months later, with immigration reform stalled in Congress, the numbers started climbing again and have now returned to average. That comes to about half a million immigrants arrested a year. A great deal has changed on the border. Because of increased enforcement and control of the drug cartels on the Mexican side, human smuggling has developed to an industrial scale. Illegal immigrants, in the hands of professional smugglers, find themselves trapped in a system of cruelty, neglect and death.

There was no reason to notice the trailer in Frio County, Texas—except for the voice of a woman crying, "we don't want to die."

In 2015, the sheriff freed 39 men, women and children overcome by heat. They were rushed to medical treatment and, this time, no one died.

18-wheelers packed with people are discovered at a rate of more than 100 a year just in Texas. Last July, this one was found in San Antonio with well over 100 Mexican and Central and South American migrants inside.

Cale Chambers: It was eerily quiet. When the doors opened, I expected to see people standing. All we saw was people laying down.

Paramedic Cale Chambers reached for unconscious victims.

Cale Chambers: Extremely hot to the touch.

Scott Pelley: Physically hot to the touch?

Cale Chambers: Physically hot to the touch. People at the brink of death that were at the end of their rope and then people that were alive but declining as we were there.

Scott Pelley: You were losing them.

Cale Chambers: Sure, yeah.

Cale Chambers describes treating migrants smuggled into the U.S. in the back of an 18-wheeler CBS News

The trailer was designed to be refrigerated so it was sealed tight. The cooling system was broken. 10 died including two children. 29 were critically ill.

Jeremy Slack: They're doing it out of a sense of desperation. People simply fear for their lives and they have no other way of surviving.

Jeremy Slack is a researcher who has spent years interviewing immigrants in Mexico. He's a professor at the University of Texas at El Paso.

Scott Pelley: What is so terrible in Central America and in Mexico that it drives this migration?

Jeremy Slack: Well we have intense levels of violence – both in Central America and parts of Mexico where the population has been targeted in a way that we had never seen before. Issues such as extortion are one of the main drivers for immigration because gangs and drug cartels start extorting businesses which eventually leads to the business being forced to close down. And now not only do people have no economic sustenance, but they also have people trying to kill them. And those two factors are incredible drivers of migration.

We met some of the immigrants when they surrendered to the Border Patrol. A 16-year-old girl told us that she was threatened with rape by a gang in El Salvador. This boy journeyed 1,000 miles from Guatemala, alone, hoping to reach his parents in Florida. They ended up in detention where they can apply for asylum or eventually be deported.

CBS News

This traditional route – over the Rio Grande river and through the brush on foot – is the path smugglers often use to funnel immigrants to the 18-wheelers on the U.S. side. But many are lost here.

Mike Vickers: My wife came home from the grocery store at 5:00 one afternoon, our dogs were playing with something in the yard and it was a human skull.

Mike Vickers' South Texas ranch lies on the smugglers' routes.

Mike Vickers: I probably got 500 pictures of different bodies. We didn't find all of those, some of 'em were found by ranch hands, sheriff's department, different people.

Scott Pelley: 500 over what period of time roughly?

Mike Vickers: Since about 2004.

Scott Pelley: What's killing them?

Eddie Canales: The heat and being unprepared.

Eddie Canales works in the same county as Mike Vickers' ranch.

He crossed over through Piedras Negras?

In 2013, Canales founded the South Texas Human Rights Center which helps rescue endangered immigrants and helps identify the dead.

"They wanted water. There were some people saying that they wanted to die. I heard a mom scream for her children."

Scott Pelley: We came across the bodies of two men who apparently froze to death during a cold snap the other day.

Eddie Canales: They were young men. They were 18 and 19-year-olds. One, one was from Mexico and one was from El Salvador.

Scott Pelley: How often are bodies found around here?

Eddie Canales: Last year, 61 bodies were recovered.

Scott Pelley: That's the ones you know about.

Eddie Canales: That's the ones we know about. The sheriff here will tell you that for every one recovered there's five still out there.

Of these survivors, some are led by smugglers to safe houses like these on the U.S. side which were filmed by the border patrol. In days or weeks their numbers grow until there is a truckload. The migrants aren't told about the 18-wheelers until it's too late and then they are forced to board. We wanted to understand their desperation so we traced a survivor of the fatal San Antonio truck, 650 miles to his home in Aguascalientes, Mexico. 42-year-old Jorge de Santos Aguilar was pulled from the truck, unconscious. He was in a coma nearly three weeks and in the hospital nearly two months.

Scott Pelley: You have a new little boy to support?

De Santos: Si.

Scott Pelley: Was he one of the reasons that you went to America?

De Santos/Pelley Translation: Yes, de Santos told us. I do it for him.

Nearly half of Mexicans live in poverty. De Santos is married with three children in a small apartment. In Aguascalientes, he can make up to $300 a month, which doesn't pay the bills. In America, it's $5,000 a month. He's made the trip four times – worked in a factory, on a hog farm and helped rebuild New Orleans after Katrina. For his last, nearly fatal trip, he sold his truck, saved money from his past trips and paid smugglers $6,500.

De Santos/Pelley Translation: It was completely dark. De Santos told us about the trailer. There was no window, there was no light, there was nothing.

Jorge de Santos Aguilar CBS News

It's estimated the 100 and more victims in the back of the San Antonio truck, baking in their own heat, pushed the temperature well over 120 degrees – which led to the 10 deaths and 29 critically ill.

De Santos/Pelley Translation: I heard a lot of people screaming, de Santos said. They wanted water. There were some people saying that they wanted to die. I heard a mom scream for her children.

The torment lasted three hours.

De Santos/Pelley Translation: The last thing I remember, he told us, was calling out to God.

"The possibility of us catching every single thing to come through this checkpoint is just – not feasible."

Scott Pelley: Is it more dangerous today than ever?

Jeremy Slack: I would say so. There is so much enforcement in the areas that people were able to cross safely, it has pushed people more and more into places that are dangerous.

Scott Pelley: How much of this illegal immigration is controlled and run by the drug cartels?

Jeremy Slack: They're kind of the regulatory mechanism. And they essentially set the rules, so to speak, for illegal activities in the region. It has led to this professionalization, this need to collaborate and coordinate with the drug cartels because they are the ones that are able to control how officials work. They know more about sophisticated ways of avoiding apprehension, avoiding enforcement.

Scott Pelley: The drug cartels own the border.

Jeremy Slack: Definitely.

Once migrants are over the border, their next challenge is, effectively, a "second border" of federal checkpoints. On major routes, far north of Mexico, the Border Patrol operates a second set of screening stations. We visited one of the busiest 29 miles north of the border on Interstate 35.

Scott Pelley: That truck that was found in San Antonio came through here.

Jason Owens: It did.

Jason Owens is the deputy chief at the Laredo checkpoint.

Scott Pelley: How did it manage to get through?

Jason Owens: It's unfortunate, but the possibility of us catching every single thing to come through this checkpoint is just – not feasible.

Scott Pelley: The driver had his commercial license revoked.

Jason Owens: Yep.

Scott Pelley: He came through here without a license.

Jason Owens: Uh-huh.

Scott Pelley: How is that possible?

Jason Owens: So the agent on primary has just a couple seconds, given the amount of traffic that comes through and so the agent, whenever they talked to the driver, didn't have that reasonable suspicion.

Scott Pelley The X-ray was broken down that day?

Jason Owens: Yes.

The Border Patrol wanted to show us the X-ray machine, but it was broken when we were there too.

Traffic at the southern border CBS News

I'm going to go back and scan the other side.

When the X-rays work, they illuminate the horror. There were 200 people in this trailer.

When hi-tech fails, dog-tech is ever reliable.

We watched two illegal immigrants sniffed out from behind the airfoil on the roof of a rig. Chief Owens told us that they would catch many more trucks, but there are just too many.

Jason Owens: 1.3 million of these vehicles comes through here, just cargo alone, every year. Another 1.9 million passengers.

Scott Pelley: In just this station--

Jason Owens: This checkpoint alone. If this were a port of entry this would be about the third-busiest point of entry in the entire country.

Scott Pelley: If you checked them all, commerce would stop?

Jason Owens: Right. So part of our job at CBP is to facilitate legitimate trade and travel, at the same time, securing our borders.

Smugglers recruit American drivers because they are less suspicious. We wondered how they find willing Americans, so we called one.

Former truck driver Troy Dock is in a prison we were not allowed to visit. He told us he crossed the border to see the sights. A man befriended him and asked Dock to smuggle an abused woman and child across the border. After dinner and drinks the man confessed that what he really wanted was to pay Dock $5,000 to transport a dozen illegal immigrants waiting at this safe house in the United States. When Dock arrived there, the dozen turned out to be 50.

Scott Pelley: Did you have any trouble at the federal checkpoint?

Troy Dock: No, they just waved us through.

Hours later, Dock reached Dallas but two of his captives did not.

Troy Dock: They say two of 'em had passed away from a heatstroke, and the other one I think was in a coma or somethin' like that.

Scott Pelley: How long are you supposed to be there in the federal prison now?

Troy Dock: 'Til 2036.

The driver in the San Antonio deaths, James Bradley, pleaded guilty to transporting immigrants resulting in death and he will be sentenced later this month.

Mike Vickers: More Border Patrol agents. That's what we need here. We need at least another 150 agents here in Brooks County.

South Texans including Mike Vickers are improvising. Vickers organized the Texas Border Volunteers – 300 armed civilians who patrol ranchlands and call in smuggling activity. The volunteers have no legal authority. And they were investigated by the sheriff in 2014 for detaining and tying up illegal immigrants while waiting for the border patrol – something that Vickers says they won't do again.

Scott Pelley: Some people watching this interview are saying to themselves right now, "He's an armed vigilante taking the law in his own hands."

Mike Vickers: We've heard that before. This is a massive invasion. We've been doing this for 11 years, there have been thousands of people that we've reported that otherwise would have gotten in – came here scot-free.

Eddie Canales, the founder of the South Texas Human Rights Center, is focusing on rescue. He's set up more than 100 water stations.

Scott Pelley: You know there are people who say you're encouraging illegal immigration by making it possible to get through here.

Eddie Canales: Well I don't think I'm the overriding factor of why people come here, you know? There's people that are leaving their countries by being pushed out, you know and they have no choice. I'm providing humanitarian effort, and, and, you know, so people don't die, and that people don't suffer.

Produced by Ashley Velie. Associate producer, Dina Zingaro.