GEOFF BENNETT: Last night, we reported from Mexico on the increasingly global migration making its way through that country to the U.S. southern border.

And, tonight, Amna Nawaz continues that journey starting in Green Valley, Arizona.

How often do you do this drive?

PASTOR RANDY MAYER, Green Valley-Sahuarita Samaritans: I do this at least once a week.

AMNA NAWAZ: OK.

It's about an hour-and-a-half drive from Pastor Randy Mayer's church to the rugged border between Arizona and Mexico in the Sonoran Desert.

PASTOR RANDY MAYER: It's a time-honored tradition of giving help to the stranger.

AMNA NAWAZ: Mayer is the founder of Green Valley Samaritans.

And for nearly a quarter-century, he's been providing aid to migrants.

PASTOR RANDY MAYER: A year ago, we were running into groups out in the desert, but it was eight, 10, 15 people a day.

Now we're out there some days and we're running into 300, 400.

AMNA NAWAZ: Three or 400 people a day?

PASTOR RANDY MAYER: A day yes.

AMNA NAWAZ: But it's not just more people.

It's where they're coming from.

PASTOR RANDY MAYER: Hey, do you guys need anything?

MAN: Bangladesh.

AMNA NAWAZ: Bangladesh.

(SPEAKING IN FOREIGN LANGUAGE) MAN: (SPEAKING IN FOREIGN LANGUAGE) AMNA NAWAZ: Within minutes of reaching the border wall, Pastor Randy is out of the truck and handing out granola bars and water to a group of men from Bangladesh.

They have been traveling for three months and just arrived at this makeshift border camp.

Before long, more arrivals.

They're from Guatemala, finding the Samaritans after walking for five hours.

The group, including a 5-year-old, had been traveling for a month.

A five-minute drive further down the border road, another large group that recently crossed, including this father and his children from Senegal.

PASTOR RANDY MAYER: Six months ago, we had never seen somebody from Bangladesh or Africa in this part of the desert.

It was all Mexicans and Central Americans, and sometimes it would be a few people from South America, but now this is pretty regular.

AMNA NAWAZ: I notice one of the first things you say is welcome.

PASTOR RANDY MAYER: Yes, we want to make sure that at least their first encounter is one of great welcome and saying we are glad you are here.

AMNA NAWAZ: Among the group are three sisters, 13-year-old Nicole, 14-year-old Genesis and 8-year-old Valeria.

They're traveling alone from Chiapas, the southernmost state in Mexico.

Did you see any other kids traveling alone along the way?

NICOLE, Mexican Migrant (through translator): No.

Most just had their families or their parents or their aunts and uncles or even close cousins.

Everybody had somebody.

AMNA NAWAZ: And you had each other?

NICOLE (through translator): Yes, just us three.

AMNA NAWAZ: The sisters are heading to reunite with their mother, their sole provider, who came to the U.S. 7.5 years ago to support the family.

Over the phone, she shares her relief that her daughters made it safely across the border.

WOMAN (through translator): I was desperate and I was so scared because I didn't know what was going on.

AMNA NAWAZ: The girls, like everyone here, are now just waiting to turn themselves into Border Patrol.

Virtually all will seek asylum in the U.S., a legal protection that takes years to formally determine.

But, for now, with a snack, water and reassurance from Pastor Randy that transport would eventually come, spirits seem high.

But in this remote stretch of the Sonoran Desert, the journey can quickly turn dire.

It's 2:00 in the afternoon.

I just met this group of folks who just crossed over earlier this morning, they said, in the middle of the night.

Two are from Mali.

One is from Mauritania.

One is from Guinea.

Two of them are very, very sick.

They said that they have been waiting for Border Patrol to arrive for hours, and it looks like they have now arrived.

So they will start to get some medical help and see what happens next.

An hour passes.

The group is taken by ambulance to a hospital more than 90 minutes away.

It's these moments that show why this border is the deadliest land migration route in the world.

In Arizona's Pima County, more than 3,000 migrants have died in the last 30 years.

PASTOR RANDY MAYER: This shouldn't be so deadly.

By our people being out here, we literally are saving lives day in and day out.

AMNA NAWAZ: Along the same span of rugged terrain, Stephen Cristinzio with U.S. Customs and Border Protection echoes that sentiment.

STEPHEN CRISTINZIO, U.S. Customs and Border Protection: When the organizations are pushing family groups across in the middle of nowhere, that's been a big challenge.

AMNA NAWAZ: When you say the organizations, you mean the cartels?

STEPHEN CRISTINZIO: The cartels.

The cartels.

The cartels control everything here.

AMNA NAWAZ: Cristinzio helps lead Border Patrol's Tucson Sector, which covers 262 border miles in Arizona.

STEPHEN CRISTINZIO: Tucson Sector is the most inhospitable terrain on the Southwest border.

AMNA NAWAZ: I mean, how does that impact your resources?

You have got larger groups, more people coming across and more remote crossings.

What does that mean for your team?

STEPHEN CRISTINZIO: It makes it incredibly difficult for us to respond in a quick manner.

For us, the number one thing with the give-up groups is safety and preservation of life.

We triage things in that manner.

AMNA NAWAZ: Driving that border with Agent Cristinzio, we quickly get a sense of what his team is managing.

This group of 17 migrants, all Mexicans, crossed the border just moments earlier through a broken section of the border fence.

The pace of what Border Patrol calls give-ups, or people turning themselves in, is relentless.

Apprehensions in this part of Arizona were up 30 percent in March compared to a year ago and just down from record levels a few months ago.

STEPHEN CRISTINZIO: Probably can't get them all in, right?

AMNA NAWAZ: The group is loaded onto a van to be taken to a nearby facility in Nogales, Arizona.

STEPHEN CRISTINZIO: We will get a medical screening.

Basic biographical information will get taken.

That will happen first.

Yes, they're coming across right there.

AMNA NAWAZ: Before Cristinzio can finish his thought, he sees another group approaching.

Do you have space, staff, resources to support all of them?

(LAUGHTER) STEPHEN CRISTINZIO: Yes, we might - - I don't know when we can get the next van.

It might be a little bit before he comes back, and then... AMNA NAWAZ: Over the course of an hour, we document at least 70 people, minors, families, children as young as 6 months, crossing the border to turn themselves in at this one remote spot.

But it's not just asylum seekers.

Agent Cristinzio says a masked man on the border fence is likely a scout for a cartel.

STEPHEN CRISTINZIO: He will watch us.

If it's people or drugs, whatever that guy is looking to push across, he's just trying to tie up our resources, get us out of the area.

AMNA NAWAZ: In fact, moments earlier, agents on ATVs marched two men out of the brush who'd been evading Border Patrol.

It's a sight that fifth-generation rancher Jim Chilton says he's increasingly seeing at his ranch, which spans 50,000 acres, including 5.5 miles along the Mexico border.

JIM CHILTON, Cattle Rancher: Through the Obama and Trump administration, averaged about 230 people coming through the ranch.

In the last three years, it's been at about 1,200 people.

AMNA NAWAZ: Chilton tracks the solicit traffic using motion-activated cameras on his property.

What do you think should be done to stop those guys from cutting through your ranch?

JIM CHILTON: President Biden made a huge mistake stopping the wall.

The wall is a very helpful tool.

It would require the Border Patrol being at the wall, and anybody trying to cut it or crawl over it or under it, they apprehended them and kick them back into Mexico.

AMNA NAWAZ: He says he supports more legal immigration pathways, but the current flow of people claiming asylum between ports of entry is untenable.

JIM CHILTON: I feel for the undocumented immigrants.

They're just trying to get into the country.

But we can't accept everybody that has an issue.

AMNA NAWAZ: A lot of folks on the outside looking in say there's a siege going on, there's an invasion going on at the border.

Is that how you see it?

JIM CHILTON: When you have eight to 10 million people coming into the United States, one could almost say it's an invasion.

AMNA NAWAZ: But opinions on this frontier differ.

Chilton's neighbor, just down the road, doesn't see it exactly the same way.

Cattle rancher Lori Lindsay's property also runs along the Mexico border.

LORI LINDSAY, Cattle Rancher: It's super peaceful.

It's beautiful.

I have lived in nine states.

This is my favorite place.

AMNA NAWAZ: And this comes with the view of the border wall.

(LAUGHTER) LORI LINDSAY: It does.

AMNA NAWAZ: Right here.

LORI LINDSAY: Oh, I hadn't noticed.

AMNA NAWAZ: Yes.

(LAUGHTER) AMNA NAWAZ: Lindsay says she's never felt threatened here.

And she doesn't put much stock in the immigration debate among people thousands of miles away.

LORI LINDSAY: If you're not familiar with the border, it sounds very scary.

It sounds like we're being invaded, criminals are coming in to get us.

And it's just not true.

I mean, not that there isn't a problem.

There is a problem with the cartels.

We need to deal with those.

But there are two separate issues going on.

You have got that, and then you have got a mother who's come seven countries away with her young children.

I think, how desperate are you?

Because I wouldn't want to do that with my kids.

AMNA NAWAZ: Back at the border, Division Chief Stephen Cristinzio and Border Patrol agents are on the front lines of disentangling these two issues and managing a humanitarian mission that many Border Patrol agents didn't necessarily sign up for.

Do you worry, after seeing hundreds of people a day, thousands of people every week, that there's compassion fatigue, burnout?

STEPHEN CRISTINZIO: I don't think so.

It's hard to look at a group like this and see all these little kids.

Those kids didn't ask to be here.

And you can put yourself in that situation.

Imagine you having to make those sacrifices, make those decisions to travel hundreds or thousands of miles with that little girl right there.

She's probably 9 months old.

AMNA NAWAZ: Maybe a year.

STEPHEN CRISTINZIO: Maybe a year.

And so, no, I don't worry about compassion fatigue.

I don't.

AMNA NAWAZ: Pastor Randy and the Green Valley Samaritan volunteers say they're frustrated by elected officials' failure to act.

PASTOR RANDY MAYER: For over 30 years, our politicians, both the Republican and the Democrats, have not chosen to do anything but throw money at security And it hasn't changed a single thing.

In fact, it's probably brought more people here.

AMNA NAWAZ: As the sun starts to set, dozens of migrants wait for Border Patrol to arrive, including Sisters Genesis, Nicole and Valeria, eagerly anticipating seeing their mother for the first time in more than seven years.

GENESIS, Mexican Migrant (through translator): I'm very excited to see her.

It's been so long since I have seen her.

AMNA NAWAZ: But, after we leave, that excitement turned to worry.

In a moment of confusion, as night fell over the desert, the girls are left behind by a Border Patrol van.

Their mother alerted "NewsHour."

We alerted Border Patrol, who carried out an overnight rescue of the girls.

Last week, the sisters reunited with their mother, three more survivors of a dangerous journey who've reached their next safe space, whatever the path ahead.