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Under Biden, crossing the border has become like a lottery for migrants. Timing is everything.

"Sometimes I ask myself why they [let me stay] and they deported others," said a 20-year-old Nicaraguan man. "And I give thanks to God."

McALLEN, Texas — As dawn broke over a shuttered flea market in Mission, Texas, on May 25, Anna and her 7-year-old son, Walter, sat sobbing on the curb. Anna and Walter, of El Salvador, had once again been caught by the Border Patrol trying to enter the U.S. undetected, and they feared a repeat of what happened the last time they were caught.

In April, U.S. border agents had sent them back into the Mexican city of Nuevo Laredo, where, Anna said, they were kidnapped and held for $10,000 ransom.

"I'm afraid that they are going to send me back to Mexico," she said. "I don't want to go back. I am afraid there. I am very afraid." Walter huddled next to his mother, clutching his stuffed dinosaur, named Señor Dinosaurio, as he cried.

Anna and her 7-year-old son, Walter, who holds a stuffed dinosaur, said this is their second time being apprehended by Border Patrol. They were sent back to Mexico the last time, and they said they fear being sent back again after they were kidnapped there a month ago.Abraham Villela / NBC News

While they sat on the curb, waiting to find out what U.S. officials would do with them, 15 single adult men who had been caught crossing the border a few days earlier were released to a nearby shelter, where they waited for flights and buses that would take them to cities across the U.S.

There they would reunite with family members and wait to have their asylum cases heard by immigration judges. They knew they were among the lucky ones.

"Sometimes I ask myself why they had me and they deported others," said a 20-year-old Nicaraguan, who said he left home to find work and was now headed to Miami. "And I give thanks to God."

When he took office, President Joe Biden loosened rules at the border, letting children without parents cross — but agents were supposed to expel all other undocumented migrants.

The policy allows the Biden administration to say, "The border is closed."

In reality, the border is not closed. Under Biden, the determination of who stays and who goes has become a lottery with winners and losers. Timing is everything, the merits of an asylum claim often beside the point. On some days here in the Rio Grande Valley, the busiest section of the U.S.-Mexico border, families like Anna and Walter are expelled, while on others, single males who've come looking for work are allowed to stay pending their hearings.

The reason, said Brian Hastings, sector chief of the Rio Grande Valley Border Patrol, is that some enter on days when Mexico cannot take them back.

Hastings said 17 percent of all families and single adults apprehended by his agents this year have been released into the U.S., much as they would have been before the Trump administration began using the Covid-19 restrictions known as Title 42 to begin denying access to asylum-seekers last year.

Borderwide, about 15 percent of single adults and 65 percent of families are released into the U.S. rather than expelled, according to Customs and Border Protection data from April.

Hastings said they are released not because of official Biden administration policy but because Mexican authorities refuse to take back more than a certain number each day.

"When they run out of shelter space, a lot of times they were telling different Border Patrol sectors, 'No, we can no longer take any additional people, because we don't have additional housing or we don't have additional space in a lot of our facilities,'" he said.

Families and small children from Central America and Mexico sit in a field near the Rio Grande river after being apprehended by U.S. Border Patrol agents on on May 25, 2021.Abraham Villela / NBC News

It has been known that Mexico's capacity issues triggered the release of some families. But NBC News witnessed firsthand how the arbitrary nature of the practice can put vulnerable people in dangerous situations while granting relief to others seemingly for no other reason than that they crossed the border at the right time.

Hastings said he thinks the way Title 42 is being enforced makes it difficult to send a consistent message to would-be migrants about what will await them if they decide to make the journey to the U.S., which he strongly discourages.

A spokesperson for the Department of Homeland Security acknowledged that its ability to expel migrants can be limited by "Mexico's ability and capacity to receive those individuals."

"Individuals who are not able to be expelled are placed in immigration proceedings pursuant to the law," the spokesperson said. "The Biden administration has made it clear that our borders are not open, people should not make the dangerous journey, and individuals and families are subject to border restrictions, including expulsion."

Risking it all

Still, every day, hundreds, even thousands, of migrants follow "coyotes," or human smugglers, onto U.S. soil, some willingly turning themselves into the Border Patrol and others hoping to make it through undetected.

By 9:30 a.m. Tuesday, the Border Patrol in the Rio Grande Valley had already apprehended a group of 101 migrants from many countries — El Salvador, Ecuador, even Romania — on a dirt road close to the Rio Grande. Little children sat on the ground, exhausted and covered with dust, their heads buried in their hands. Pregnant women cupped their bellies and arched their backs on the side of the road, trying to rest after the long journey.

Abandoned shoes floated in puddles, and colored plastic bracelets littered the ground. The coyotes give migrants the bracelets to track their customers.

"It's very difficult in my country," said Sara Judith, a Honduran woman who had been apprehended with her 10-year-old daughter. "There's nothing."

Sara Judith and her 10-year-old daughter, Marleni Noemi, traveled over a month from their home country, Honduras, to the U.S. They were apprehended by Border Patrol agents on May 25.Abraham Villela / NBC News

She said she was happy to finally be in the U.S. but didn't know that families like hers could be sent back into Mexico.

The temperature was already close to 90, and it would climb to the mid-90s. And the hottest part of the year is yet to come.

Border Patrol Agent Brandon Copp, whose beat includes hundreds of thousands of acres of private ranchlands across Kenedy and Brooks counties, worries that the rise in temperatures, combined with the 20-year high in overall border crossings, will mean more deaths in this area.

As lead coordinator for CBP's Missing Migrants Program, Copp is already responding to one to two reports of dead bodies found in the Rio Grande Valley sector every week.

He said rescues of migrants in distress are up by 150 percent year over year, while deaths are up by 58 percent.

"As a Border Patrol agent, I never thought I would be going to mortuaries and doing identifications," Copp said. Just last week, he said, he had to remove the skin off the finger of a deceased migrant to run his fingerprint so the man's family could be notified about his death.

Copp and his team are busy installing 15 more rescue beacons to double the number throughout the Rio Grande Valley border sector where migrants can call for help.

Border Patrol Agent Brandon Copp, lead coordinator of CBP's Missing Migrants Program.Abraham Villela / NBC News

But not everyone calls for help in time. Copp said the human smugglers whom migrants pay to take them on the dangerous journey often tell their clients after they cross into the U.S. that they simply have to walk an hour before they'll be in a major city, like Houston, and then leave behind anyone who can't keep up. He has even heard of smugglers' throwing cellphones so dying migrants won't be able to call for help and identify who left them there.

"We will see more deaths. And that's the sad truth for us," Copp said.

Immigration advocates also say uncertainty around Title 42 is driving many migrants to take more dangerous routes to avoid being apprehended altogether.

"The Biden administration's retention of Title 42 and refusal to open the legal ports of entry is having the perverse effect of forcing desperate asylum-seekers fleeing danger to cross between the ports, which is to nobody's benefit," said Lee Gelernt, deputy director of the Immigrants' Rights Project of the American Civil Liberties Union, who is a lead plaintiffs' lawyer in a lawsuit challenging the use of Title 42.

For now, the Biden administration has made no promises of end dates for the Title 42 policy, even as Covid-19 restrictions ease across the country. Homeland Security Secretary Alejandro Mayorkas has said the policy is in place to protect both migrants, who would need to be kept temporarily in congregate care settings if allowed in, and agents.

Gelernt said the policy of guaranteeing entry to only unaccompanied children forces some families to self-separate to give their children the best chance to seek asylum.

Elsa Sinche Tenelema, from Ecuador, began to cry as she recalled her anguish over whether to send her daughter Kelly to the U.S. alone.

Kelly, 6, has a congenital ear deformity and impaired hearing. Sinche keeps Kelly's hair in tight braids so she can rest the arms of her glasses in her hair, because her ears will not hold them. Sinche brought her daughter to the U.S. because she had heard that a doctor in California could operate and enable Kelly to hear fully for the first time.

She said that other people told her to send Kelly alone and that she knew it was a risk to accompany her. They were allowed to stay and were given a date for an asylum hearing in two months. Outside a migrant shelter in McAllen, Sinche said she had been worried that she was making the trip in vain but that now she is hopeful she will win her asylum case.

"God willing, they will help us so that we can stay longer and fulfill the dream we came for," she said.