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LUKEVILLE, Ariz. — They crossed just before dawn. And there was little to stop them.

At around 5 a.m. last Monday, a group of 84 migrants hurried off a large tour bus on a Mexican highway, crawled under a small fence and walked into the United States.

Customs and Border Protection videotaped the migrants, all families from Central America, crossing the border in Quitobaquito Springs in Arizona’s Organ Pipe Cactus National Park, a four-hour drive from Tucson.

Border agents used a mobile surveillance capability truck to capture images of the crossing, released exclusively to NBC News.

Fernando Grijalva, CBP’s patrol agent in charge of the area where the migrants were dropped off, said his agents use the cameras to monitor larger expanses of the border.

“He saw a bus stop on the Mexican side of the border,” Grijalva said. “The bus driver got out, opened the doors and then proceeded to have approximately 80 people exit the bus and cross into the United States illegally.”

Grijalva, who has worked with CBP for over 30 years, says this incident highlights what he calls a crisis on the southern border unlike anything he’s seen before.

“What’s changed is the dynamics of the groups,” Grijalva said. “Now it’s a lot of unaccompanied children or family groups.”

In the past, he said, border agents apprehended Mexican males crossing the border to find seasonal work.

Whether this amounts to a “crisis” has become a central issue in the partial government shutdown, now entering its fifth week.

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Along the Southwest border, the overall number of apprehensions has plummeted over the last two decades. In the Tucson sector, the number has dropped from more than 616,000 in 2008 to about 52,000 in 2018.

Jeffrey Self, the Border Patrol’s acting chief of the Tucson sector, insisted a humanitarian crisis had developed. Although the overall number of apprehensions was down, families are more difficult to for his agents to process and diverted resources from stopping drug smuggling and other crime.

The nearly 20,000 border agents stationed across the country are currently working without pay.

“We're an apolitical organization,” Self said. “We execute our orders from the executive office … I can only tell you from focusing on field operations that border security is imperative to this nation.”

Another video released by Customs and Border Protection shows the largest single group of asylum seekers ever to cross into the U.S. beneath the border wall near San Luis, Arizona.

A record 376 migrants, 179 of them children, were seen going both under and over the border fence. Some burrowed underneath the barrier. All of the migrants voluntarily turned themselves into CBP after crossing into the U.S. on Monday, according to the agency.

In a tour Friday of CBP’s Tucson Processing Center, NBC News saw about 250 migrants being held in cinderblock cells with thick glass windows that overlooked a central bullpen, where CBP agents worked to process them.

Most of the 376 Central Americans, largely from Guatemala, arrested in southwest Arizona Jan. 14 used short holes dug under a barrier to cross the border in multiple spots about 10 miles east of San Luis, Arizona, according to the Border Patrol. U.S. Customs and Border Protection / AP

The asylum seekers were separated into cells: men, unaccompanied minors, women and mothers with children. Christopher Defreitas, who runs the center for CBP, said the cells are configured based on the type of migrants coming through their coordination center.

Three Coast Guard officers — also working without pay — were providing medical care to migrants inside the processing center who requested medical attention. Department of Homeland Security Secretary Kirstjen Nielsen deployed the U.S. Coast Guard’s medical corps to the southern border last month to screen immigrants after two young children from Guatemala died in CBP custody.

In all CBP facilities, agents said it works to process people as quickly as possible and provide basic medical care. Detainees eat, sleep and use the bathroom in the same room. Silver Mylar blankets were dispersed on the floor atop green mats to sleep on.

In one cell, several boys were jumping atop the divider separating the toilet from the rest of the cell.

Defreitas said that at any given time, CBP agents could be giving a pacifier to a crying toddler detained in one cell, while processing a convicted murderer in another.

In another cell, one girl around 7-years-old was crying. A CBP officer took the girl out of her cell to see an adult family member, who was pulled from the adult male cell.

The girl gave him a hug, and he kissed her on her cheek as they sat on a bench outside his cell.

Ronald Bellavia, CBP’s acting deputy chief of the Tucson sector, said CBP works with Central American consulates to confirm whether detainees who claim to be in a family unit are related. Bellavia said border agents are seeing an increase in adults crossing into the U.S. with children as a way to manipulate the system and increase their chances to qualify for asylum.

Just two days after the group of 376 people crossed near Yuma, the Border Patrol found another large group of migrants in New Mexico. A total of 247 people, including unaccompanied minors, immediately requested medical care after surrendering to authorities for processing.

Critics of the Trump administration have said its policies are now driving migrants to make more dangerous journeys to remote areas to surrender and apply for asylum. The president’s supporters say that’s why more barriers are needed in these remote places — to deter people from attempting to cross in the first place.

Regardless of the politics behind the shutdown, thousands of Border Patrol agents are still working, with or without pay.