MATAMOROS, Mexico — The stench is overpowering. During the day, it seems to bake on the squalid concrete. At dawn, it seeps into the cool air — a suffocating mix of human waste and campfires.

Just steps from Brownsville, Texas, a makeshift tent city is growing next to the international bridge. More than 1,200 migrants — many from Mexico and Central America, others from Cuba — are waiting.

This year, the Trump administration enacted what it calls Migrant Protection Protocols, or MPP. Also known as the “Remain in Mexico” policy, it requires U.S. asylum-seekers to stay in that country while their claims are processed. Before MPP, families would be allowed to wait for their court hearings in the United States.

More than 55,000 migrants have been returned to Mexico under this policy, U.S. Customs and Border Protection officials said — and it’s become a bottleneck at the border.

“This is 100 percent a humanitarian crisis,” Jodi Goodwin, a Texas immigration attorney, said. “These policies are not implemented in a vacuum and there are very real human consequences.”

Carlos, from Honduras was among the migrants who spoke with NBC News and asked not to have his last name used for fear of reprisals. The 27-year-old said he’d been at the makeshift camp for four months with his 2-year-old epileptic son — and he’s struggled to find medical care.

“The most difficult part is when my son has convulsions and I’m alone in the tent,” he said. “It’s happened twice at night and I can’t do anything.”

"We're sending a message"

According to CBP, apprehensions at the Southwest border have plummeted from 144,116 in May to 45,250 in October. That’s a 68 percent drop.

“Migrants can no longer expect to be allowed into the interior of the United States based on fraudulent asylum claims,” Mark Morgan, the acting commissioner of CBP, said at a White House briefing last week. “We’re sending a message to their criminal organizations to stop exploiting these migrants.”

The Trump administration has argued the change is working because in essence, the Remain in Mexico policy has served as a deterrent for migrants as well as human smugglers.

But immigrants advocates argue that claim is dubious and has merely increased desperation and fear on the Mexican side of the border.

In Matamoros, the Mexican government recently opened a shelter about a 30 minute walk from the international bridge in response to the influx of migrants. But many of the families refuse to stay there because they fear a growing threat from the cartels.

One man, Josué, told NBC News his two young daughters were sexually assaulted by a man he believes was a cartel operative. The girls had been washing themselves in the Rio Grande when he touched them, Josué said. He showed NBC News a police report he’d filed.

“Matamoros is controlled by the cartels and the bad people,” he said. “When I got here, I was really scared.”

So volunteers are taking action. Every day, a group called “Team Brownsville” is among those who bring food and supplies across the border.

As the sun begins to set, migrant families line up for a meal.

“It breaks my heart to see the need here,” said Mary Vanderhoof, a volunteer from New Jersey. “There’s no reason that people should be living like this.”

Sergio Córdova, one of Team Brownsville's organizers, said he’s been coming here since the summer of 2018. What started as just a few migrants with donated cots has exploded into a full-blown tent city.

“How can you look away?” he asked. “Are we going to be a country that says ‘We looked away?’ Or did we do something?”

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