In a dusty lot along the U.S.-Mexico border fence, a single Border Patrol agent was stuck with few options and falling temperatures.

A group of 64 parents and children had waded through a shallow bend in the Rio Grande to turn themselves in to the agent on the U.S. side. He radioed for a van driver, but there were none available. By 2 a.m., the temperature was 44 degrees.

The agent handed out plastic space blankets. The group would have to wait.

Mothers and fathers swaddled their families in the silvery, crinkling sheets and clustered with them on the ground, shushing the children. They shivered in the cold wind, and the sound of crying carried on, like a broken alarm.

Groups like this arrived again and again in February, one of the coldest and busiest months along the southern border in years. U.S. authorities detained more than 70,000 migrants last month, according to preliminary figures, up from 58,000 in January. The majority were Central American parents with children who arrived, again, in unprecedented numbers.

During a month when the border debate was dominated by the fight over President Trump’s push for a wall, unauthorized migration in fiscal 2019 is on pace to reach its highest level in a decade. Department of Homeland Security officials say they expect the influx to swell in March and April, months that historically see large increases in illegal crossings as U.S. seasonal labor demand rises.

Migrant families wait for a Border Patrol van to take them to a holding facility in El Paso on Feb. 22. It was cold and windy that night, so a border agent distributed plastic blankets to the group.

The number of migrants taken into custody last year jumped 39 percent from February to March, and a similar increase this month would push levels to 100,000 detentions or more.

It was a surge in the border numbers in March 2018 that infuriated President Trump and launched his administration’s attempt to deter families by separating children from their parents. Trump stopped the separations six weeks later to quell public outrage. But the controversy the policy generated — and its widely publicized reversal — is now viewed by U.S. agents as the moment that opened the floodgates of family migration even wider, worsening the problem it was meant to fix.

While arrests along the border fell in recent years to their lowest levels in half a century, they are now returning to levels not seen since the George W. Bush administration, driven by the record surge in the arrival of Central American families.

For U.S. border agents, the strain has grown more acute, as they struggle to care for children using an enforcement infrastructure made in an era when the vast majority of migrants were Mexican adults who could be quickly booked and deported. The Central American families — called “give-ups” because they surrender instead of trying to sneak in — have left frustrated U.S. agents viewing their own role as little more than the facilitators for the last stage of the migrants’ journey. They are rescuing families with small children from river currents, irrigation canals, medical emergencies and freezing winter temperatures.

“We’re so cold,” said Marlen Moya, who had left Guatemala with her sons six weeks earlier and crossed the Rio Grande with the group of 64.

Moya’s son Gael, 6, was sick with a fever and moaning, his face streaked with tears. “In Juarez, we were shoved and yelled at,” she said, looking back across the river to Ciudad Juarez, Mexico. “We slept on the street.”

Asked why she didn’t cross during the day, when temperatures were mild, Moya said she worried that Mexican police would stop them. “We’ve already come this far,” she said.

Marlen Moya, who had left Guatemala with her sons six weeks earlier, holds Gael, 6, as they wait along with Anderson, 8. Moya said she fled Guatemala City after being threatened and robbed at gunpoint at her beauty salon.

Much of the attention last fall was focused on caravan groups, mostly from Honduras, as they reached Tijuana, Mexico, not far from San Diego. Then concern shifted to Arizona and New Mexico, where groups of rural Guatemalan families began showing up at remote border outposts. Two Guatemalan children died in December after being taken into U.S. custody, as Homeland Security officials declared a humanitarian and national security crisis.

The border deal Trump and Democrats reached last month includes $415 million to improve detention conditions for migrant families, including funds to potentially open a new processing center in El Paso. But in the meantime, families continue to arrive in groups large and small, in faraway rural areas and right in downtown El Paso.

“The numbers are staggering, and we’re incredibly worried that we will see another huge increase in March,” said a Homeland Security official who spoke on the condition of anonymity to discuss the unpublished figures.

The group by the river had landed on the no man’s land between the Rio Grande and the tall, steel American fencing. They were on U.S. soil, a place that already has a border wall.

The lone U.S. agent with the group was the only one available along that span. Drug smugglers have been using the groups as a diversion, so the agent couldn’t leave the riverbank.

No vans or buses arrived to pick up the families. Other agents were busy at the nearby processing center because so many groups had arrived in El Paso that night, and still others were at the hospital, where they were helping parents and children receive treatment for severe flu symptoms.

Homeland Security officials have been urging lawmakers to grant them broader powers to detain and quickly deport families in a search for deterrent measures. Their attempts to crack down using executive actions have been blocked repeatedly in federal court.

The migrants travel by van to a holding facility in El Paso.

The Trump administration has begun sending some asylum-seeking Central Americans back to Mexico to wait while their claims are processed, but so far that experiment has been limited to California’s San Ysidro port of entry.

About 150 migrants were sent back across the border in February, according to Mexican authorities, but that is a small fraction of the more than 2,000 unauthorized migrants coming into U.S. custody on an average day.

Homeland Security officials said Friday that the pilot program, which they call Migrant Protection Protocols, will expand to El Paso and potentially other locations in coming weeks, predicting that the number of Central Americans sent back would grow “exponentially.” Some of the cities where they will wait are among the most dangerous in Mexico.

Mexican officials are cooperating by providing general assistance and job placement for those sent back to wait, but privately they have warned the Americans that their capacity to take parents with children is extremely limited, especially families that need welfare assistance and enrollment in already-crowded public schools.

Migrants at the border hold tight to the blankets and move about the area to stay warm while they wait for the van.

Arrivals and departures

U.S. court restrictions on the government’s ability to keep children in immigration jails — and the sheer volume of people arriving — have left Homeland Security agencies defaulting increasingly to the overflow model Trump deplores as “catch-and-release.”

Volunteer workers from religious charities were visible at the El Paso airport last month, guiding newly arrived Central American families through the terminal, directing them like a tour group.

The adults wore GPS monitors on their ankles and carried manila envelopes with instructions telling them when to appear in court for their asylum claims. Some were traveling in premium seats, the only last-minute tickets available when their families arranged the flights.

It was the first time many of the migrants had been on an airplane. For Dionel Martinez, it was the second.

The 48-year-old Guatemalan came to the United States three decades earlier, working as a landscaper until he was deported — his only other time on a plane.

“We’re going to Pennsylvania,” he said. A friend had arranged a job at a pizzeria there.

With the savings from his first stint in the United States as a young man, Martinez was able to buy some land in his home country and start a family. But a drought this year had left them hungry.

“There was no harvest,” he said. “Not one grain of corn.”

His son Darwin, 13, came with him to the United States this time. The boy fainted during the journey, his father said, when they had to stand for hours in the back of a cattle truck.

Martinez said he paid 30,000 Guatemalan quetzals, about $2,500, to a “coyote” smuggling guide. It was a cheap rate, but it meant that he and his son traveled through Mexico in trucks, like cargo.

Across rural Guatemala, Martinez said, word has spread that those who travel with a child can expect to be released from U.S. custody. Smugglers were offering two-for-one pricing, knowing they just needed to deliver clients to the border — not across it — for an easy surrender to U.S. agents.

The group of 64 migrants came across the Rio Grande and presented themselves to U.S. authorities. Migrant families keep walking to stay warm as they wait for the van in El Paso. The group of 64 includes parents and children hoping to remain in the United States.

Migrant families keep walking to stay warm as they wait for the van in El Paso. The group of 64 migrants came across the Rio Grande and presented themselves to U.S. authorities.

“If this continues, I don’t think there will be anyone left in Guatemala,” Martinez joked. The men from his village near the town of Chiquimula were all leaving, he said, bringing a child with them.

Martinez said he used the family home as collateral. He had four months to pay off the $2,500. “I need a way to feed my family, and this is it,” he said.

Not all Central American families are economic migrants. Others, especially from Honduras, arrive with stories of gang threats and violent attacks. After crossing the border, a U.S. asylum officer performs a preliminary screening to determine whether their fears of persecution are credible enough to deserve a hearing with an immigration judge.

The problem, Homeland Security officials say, is that a growing portion of those who pass the initial screening never appear in court. They know asylum standards are tightening. Or, like Martinez, they have a prior deportation from the United States that all but disqualifies them from getting asylum.

Once released into the U.S. interior, some shed their monitoring bracelets and slip into the shadows to remain in the United States, a country where wages are 10 times higher than in Central America.

The saturation at the border means that it matters little whether a parent’s story of persecution is sufficiently credible; the United States has just three detention facilities appropriate for families, with about 3,000 beds, and those are full. The pipeline backs up into Border Patrol stations that were never designed for long-term detention, let alone children, many of whom arrive sick after days in cramped quarters.

A Border Patrol agent in Antelope Wells, N.M., uses his truck as a perch to spot smugglers and groups of migrants approaching the border.

A crossing gone quiet

The tiny, remote Antelope Wells, N.M., border crossing, where 8-year-old Jakelin Caal arrived in December before falling fatally ill, is now staffed with a team of medically trained Border Patrol agents. But that crossing has gone quiet in recent weeks, as more large groups turn up on El Paso’s riverbanks.

For families too poor to hire a smuggler, it was the only place to cross, converging with others who sought safety in numbers. Carlos Guevara, 35, said he and his son had wandered the streets of Juarez with nowhere to sleep, then spotted the crowd heading for the river.

“I want to give my son a better life,” he said. Guevara said he earned about $6 a day for farm labor in Honduras, and left a month earlier with, Carlitos, 7, en route to Michigan. “I can’t stand being poor anymore.”

Swathed in plastic, his son approached the headlights of an agent’s Border Patrol truck, its idling engine offering some warmth. Other children in the group were still crying and calling out.

Ramiro Cordero, a Border Patrol official assigned to accompany reporters, called on the radio, and said he would go back to the nearest station and get a van himself.

Border Patrol official Ramiro Cordero puts bracelets on Mairon Argueta and his daughter Elsi Argueta, 4, who arrived in El Paso from Honduras. The bracelets identify the group of migrants and how many people crossed.

“This is what’s happening on a daily basis,” Cordero said. “You’ve got to understand that we have to take care of everyone that comes across. And this requires transportation and a lot of logistical support.”

“Hopefully the vans can get here to transport them to one of the processing facilities,” he said. “But for right now, this is where we stay.”

He lined up the parents and children to issue bracelets to each one with a number corresponding to their arrival group. “They will be provided with basic needs, whether it’s water, juices, warm meals,” Cordero said. They would also get a medical screening.

Two blue-uniformed customs officers, summoned to help the Border Patrol agents, arrived with a van after 3 a.m.

The agents loaded the families into the vehicle, needing three trips to transport the entire group. The crying had stopped. On the radio, a dispatcher said there were already 607 migrants in custody at the processing center where they were headed, more than twice its capacity.