From South America, the Caribbean, Asia and beyond tens of thousands of migrants bound for the United States have been arriving to Mexico each month. Then their paths to the U.S. border begin to diverge.

Brazilians mostly head for Yuma, Ariz. Of the 7,366 Brazilian nationals taken into custody by U.S. agents along the border in May, 63 percent crossed into the Yuma area, the latest government figures show.

The majority of Venezuelans enter the United States near Del Rio, Tex. In May, 74 percent of the 7,371 Venezuelans who crossed the border arrived there.

Ecuadorans go to El Paso. Cubans use Yuma and Del Rio. Haitians head for Del Rio and El Paso. And the Rio Grande Valley, the Border Patrol’s busiest sector, remains the prime entry point for Central American families with children under 7.

These forking routes are part of a migration pattern that U.S. officials say they have never seen to this degree. While social media and word-of-mouth play a role in channeling some migrants toward certain crossing points, smuggling organizations are taking advantage of uneven enforcement policies to convert sections of the U.S. border into designated entry lanes for specific nationalities and demographic groups.

Since March 2020, the U.S. government has relied on a provision of the U.S. health code known as Title 42 authority to quickly return most migrants to Mexico and reduce the risk of spreading coronavirus inside border stations and jails. But the policy is not applied uniformly across the border, and smugglers are directing migrants to cross where they are least likely to be sent back.

Raul Ortiz, the deputy chief of the U.S. Border Patrol, said migrants making the journey north often look to cross in areas rumored to be safe, relying on the recommendations of family members and social media. “Then you do have the smuggling organizations that are specifically trafficking in some of these migrant demographics,” said Ortiz, in an interview.

The pattern has left some sectors of the border under strain, he said, but CBP has ramped up capacity and streamlined processing times to avoid backups. “It would be nice if all my southwest border sectors operated the same way, but it’s just not the case,” said Ortiz.

In the Rio Grande Valley, Mexican authorities do not take back non-Mexican families with children under age 7, citing insufficient shelter space, so smugglers use the area for families who fit that profile. Elsewhere along the border, Mexico does not accept returns from non-Spanish speaking countries, and limits the number of people it will take back in areas with little shelter capacity, giving migrants who cross in those areas a better shot at getting released on the U.S. side.

Criminal groups seize on those opportunities, said Cris Ramon, an immigration analyst in Washington who closely tracks CBP data. “Smugglers are recognizing the Balkanization of border policy and trying to exploit it as much as possible,” he said.

U.S. Customs and Border Protection is due to release enforcement data this week showing another uptick in the number of border-crossers taken into custody in June, the fourth consecutive month that apprehensions and detentions have surpassed 170,000.

Before taking office, President Biden said he did not want “2 million people on our border.” U.S. authorities are on pace to detain more than 1.5 million migrants during the current fiscal year that ends Sept. 30, the highest annual total since 2000.

Biden officials insist the U.S. border is closed to unlawful migration, but that claim is belied by CBP data showing declining percentages of migrants returned to Mexico under Title 42. In May, only 20 percent of migrants who arrived in family groups were returned to Mexico under Title 42, and about 75 percent of single adults were sent back or “expelled,” a lower percentage than previous months.

Following the record influx this spring of teenagers and children crossing without parents, the Biden administration has urged Mexico to increase its interior enforcement, and fewer minors have been arriving alone in recent months. The number of families arriving from Guatemala, Honduras and El Salvador — Central America’s Northern Triangle — has also been falling.

Those declines have been offset, however, by increases in migration by Mexican nationals, and the arrival of more migrants from beyond Mexico and Central America. Brazilians, Nicaraguans, Haitians, and many other nationalities are arriving by the hundreds or thousands each month, as nations around the world reel from the pandemic and the Biden administration rolls back Trump-era border controls while curbing interior enforcement.

The United States was essentially closed to asylum seekers for most of 2020, but soaring numbers of migrants are now arriving to seek humanitarian protection from countries that have not been major senders in the past.

More migrants arrived from Ecuador than El Salvador in May, records show, a trend that also complicates the Biden administration’s effort to have Vice President Harris address the root causes of migration from the Northern Triangle region.

The influx also includes some of the millions of Venezuelans who fled their nation years ago to resettle in Colombia, Peru and elsewhere, but who are now on the move again, seeing a new opportunity to reach the United States as the pandemic and its economic fallout ravage South America.

Nicaraguans are fleeing a repressive crackdown. Mexican adults are crossing at levels not seen in a generation, and they account for the majority of apprehensions in western border sectors like El Centro and Tucson.

Brazilians are flying into Mexico, where they do not need visas, and heading for Yuma. More than 4,000 Romanian migrants have entered in the past 9 months, many of them members of the Roma ethnic minority that has faced centuries of discrimination in Europe. They overwhelmingly cross in San Diego or the Rio Grande Valley, at opposite ends of the border.

Some migrants travel as clients of smuggling organizations from the beginning of their journeys, hiring guides who arrange for their travel to Mexico and all the way to the U.S. border and across it. Others are picked up by criminal groups after they arrive to Mexico or as they gravitate toward specific routes and crossing points popular on social media.

Monterrey, the largest city in northern Mexico, is a logistics hub for smuggling organizations that route migrants to stash houses along the border, before crossing them into specific sectors, current and former CBP officials say. Hermosillo, along the western route through Mexico, is a similar hub for groups bound for California and Arizona border sectors.

Roy Villareal, the retired former chief of the Border Patrol’s Tucson sector, said smuggling organizations “have the ability to funnel migrants to designated locations based on their capabilities and according to perceived enforcement operations” on the U.S. side.

A lot of smuggling recruitment happens on social media, he said, where smugglers present themselves as full-service travel agencies. Once an individual leaves home, “that individual or family is controlled from the point of departure to the border, with a clear itinerary,” Villareal said.

Erika Mouynes, Panama’s minister of foreign affairs, called the fivefold increase in migration through her country this year “untenable” in a recent op-ed for Foreign Policy.

“This surge has deeply affected small villages in Panama near the Darién Gap, which require significant investments to deal with the increasing demand for food, water, and sanitation,” Mouynes wrote, describing desperate conditions in the roadless jungle area traversed by migrants coming north. “These areas also must provide accommodation for groups of different social, linguistic, and religious backgrounds, as well as care for those who suffer from traumas such as rape and abuse.”

Local officials in U.S. border cities are making the same appeals for assistance. “They don’t have the shelter space and other infrastructure that exists in places like the Rio Grande Valley to help migrants when they are released by CBP,” said Jessica Bolter, an analyst at the nonpartisan Migration Policy Institute in Washington.

In May, the Biden administration launched “Operation Sentinel” to target smuggling organizations and their financial resources, and CBP has deployed U.S. agents and officers to airports in Panama and Mexico to help interdict migrants arriving from outside the region. Ortiz attributed the slight dip in border-crossings in May by Brazilians and Venezuelans to the U.S. enforcement presence at those airports.

Ortiz said the Biden administration is preparing to end Title 42 processing, but he would not confirm reports that would occur this month. He said authorities were developing a pilot project in the Rio Grande Valley that will increase the use of “alternatives to detention” such as electronic monitoring for what could be an even larger inflow of migrants requesting asylum in the coming months.

Defense Secretary Lloyd Austin has authorized the use of 3,000 National Guard troops who will deploy beginning in September to help CBP, Ortiz said.