Crouched low in the brush along the riverbank, Border Patrol agent Robert Rodriguez watched the Mexican side of the Rio Grande, waiting. A norteño ballad drifted from a radio somewhere on a nearby farm, and two pigs cooled themselves at the water's edge, wading to their bellies. For a moment, one of the border's busiest places for illegal crossings looked placid.

Then a raft appeared.

Within seconds it was in the water, a teenage guide steering the current while his boss, an older man, stood watch on the bank. In less than a minute, the teenager delivered a woman and a boy to the U.S. side and they climbed out, shoes sinking in the wet silt.

Rodriguez stepped onto the path to stop them, but the woman and the boy did not run. They wanted to be captured. This is how it works now.

The era of mass migration by Mexican laborers streaming into California and the deserts of Arizona is over. Billions spent on fencing, sensors, agents, and drones have hardened the border and made it tougher than ever to sneak into the United States. The migrants coming today are increasingly Central Americans seeking asylum or some form of humanitarian protection, bearing stories of torture, gang recruitment, abusive spouses, extortionists, and crooked police.

They know the quickest path to a better life in the United States is now an administrative one — not through mountains or canyons but through the front gates of the country's immigration bureaucracy.

Last year, U.S. immigration courts received nearly 120,000 asylum claims from migrants facing deportation, a fourfold increase from 2014. Those filings have pushed the number of pending cases before U.S. immigration courts to more than 750,000, collapsing the system and upending President Trump's sweeping promises to lock down the border.

This past spring, Trump fixated on a caravan of asylum seekers traveling through Mexico, about 300 of whom eventually crossed into the United States. Now, a much larger procession of as many as 7,000 Central Americans is trekking north toward the border, despite threats from the president to stop them with U.S. troops and sever aid to their countries.

Families are coming in caravans and on their own because it works. Only 1.4 percent of migrant family members from Guatemala, Honduras, and El Salvador who crossed the border illegally in 2017 have been deported to their home countries, according to Department of Homeland Security officials.

There is a sinking feeling, among DHS officials, that more caravans are yet to come and that they will only get larger.

As Rodriguez radioed another agent to pick up the woman and the boy, she handed him her Honduran identification card. Cecilia Ulloa was 25. Darwin, her son, was 13. The math took a moment to sink in, and Ulloa appeared to recognize a familiar look of confusion.

"My stepfather," she said. "It started when I was 10."

After a decade in prison for rape, her stepfather was free now, stalking them, blaming her for ruining his life, Ulloa said. "He's going to kill us."

Police in Honduras had told her there was nothing they could do, she said, so she and her son left for the United States. They wanted asylum.

Chances were they would be denied. But it could take months, or longer, for the U.S. immigration system to determine whether Ulloa and her son deserved protection. They would probably not be sent back to Honduras anytime soon.

Some migrants' stories of gang threats and police indifference have a rehearsed quality, suggesting they are concocted. The smuggling guides who charge $10,000 or more for the trip provide transportation and meals, but also coaching, including the key words migrants should say to convince U.S. asylum officers that their fears meet credibility standards.

But there are many with no need to make things up. The countries they are running from have some of the highest murder rates in the world. Their criminal justice systems barely function. Some have been victimized already.

For people like Ulloa and her son, here's how it works:

Those who cross the border and turn themselves in are interviewed by a U.S. asylum officer to determine whether they have a "credible fear" of facing persecution back home. The Supreme Court has ruled that an asylum seeker's fear is considered "well-founded" if there is a 10 percent chance they will face persecution, and those who potentially qualify are referred to an immigration judge.

Between Oct. 1, 2017 — the start of the 2018 fiscal year — and June 30, the period for which the most recent statistics are available, the government received more than 73,000 credible-fear claims, up from 5,000 during all of 2009. Of those 73,000 who were interviewed, 76 percent were found to have a credible fear of return.

The finding does not mean that a judge will eventually grant asylum. Justice Department statistics show that fewer than 10 percent of Central American applicants are awarded asylum, but the process of applying offers a shield from deportation and a toehold, however tenuous, in the United States.

Under Trump, asylum denial rates have reached their highest levels in more than a decade. But nearly half of those rulings are issued in absentia, because the applicant does not appear in court.

That is the breach Trump officials see: If asylum seekers think their case is likely to be denied, they can drop out of the court system and disappear, remaining in the United States illegally.

In the absence of a physical wall, the Trump administration is laying down new barriers to the asylum process. The U.S. immigration court system is a branch of the Justice Department, not the judiciary, and the attorney general effectively functions as a one-man Supreme Court. In June, Jeff Sessions issued a sweeping ruling that overturned the case of a Guatemalan domestic-violence victim who had demonstrated that police failed to protect her from spousal abuse and rape.

Sessions' ruling said asylum laws are meant to shelter those facing persecution for political or religious beliefs, or their membership in a well-defined social group, not those fleeing what he called "private" forms of violence.

Asylum seekers who make their claims at official border crossings — not on the banks of the Rio Grande — are not breaking the law. But U.S. agents have to let them cross the bridge first.

On a recent morning in South Texas, immigration lawyer Jennifer Harbury walked across the river into Mexico under a blazing sun, waiting for a nun to pick her up. They drove to a Reynosa migrant shelter in a bullet-scarred neighborhood full of cartel lookouts and stash houses used by smugglers to stage illegal crossings.

Harbury is an irritant to U.S. border officials as well as to the cartels. She provides free legal advice and assistance to asylum seekers, so the nuns who run the shelter call her often to see whether she can help migrant families desperate for legal advice. Harbury's pro bono work takes profits away from traffickers, because they charge a "tax" of several hundred dollars to those who cross illegally along the river. They earn nothing from the migrants Harbury escorts to the official border crossing.

Harbury is one of the activists who also help asylum seekers stranded in the no man's land on the pedestrian bridge over the river. In recent months, U.S. officers have been turning migrants away, telling them to come back later. Harbury and others have criticized the practice as unlawful, but DHS officials say that port officers have multiple responsibilities and that busy border crossings have capacity limits.

It was Harbury who provided ProPublica with the surreptitious audio recording of a child screaming for her mother that dealt a severe blow to the family-separation policy. She has absorbed the stories of thousands of asylum seekers and views her job with the urgency of an emergency responder. She intends to help as many asylum seekers enter the United States as possible, because she believes she is saving their lives.

"These people have the most horrifying stories I have ever heard," she said. "I don't think people have better claims than those running from the cartels."

The nuns had asked Harbury to help a young mother stranded for more than a week, Maria Magdalena Gonzalez, 21, and her son, Emiliano, 3. A gangster in Gonzalez's home state of Guerrero, Mexico, was threatening to kill her for rejecting his advances, she said. But when she and her son tried to approach the U.S. border crossing a few days earlier to seek asylum, they had been turned away.

With more and more Central Americans showing up at the port of entry, U.S. officers had set up an impromptu checkpoint over the middle of the Rio Grande, blocking them from setting foot on the U.S. side to start the asylum process.

Those who fail to cross are put at risk, because cartel lookouts ply the Mexican side of the bridge, watching for Central Americans who have been turned away. The migrants are prime targets for kidnapping because criminal groups assume they have relatives living in the United States with enough money to pay a ransom.

Harbury was there to make sure Gonzalez and her son weren't rejected again.

A nun drove them to the bridge over the river, and Harbury walked alongside them until a Mexican immigration official stood in the way. He had been looking for asylum seekers from Central America, but Gonzalez and her son were Mexican, so there was nothing he could do to detain them.

He told Harbury the U.S. agents had not been letting asylum seekers through, or were making others wait three or four days to be allowed to approach the American side.

Harbury, Gonzalez, and the boy continued walking until three American officers blocked them halfway across the bridge. "We want asylum," Gonzalez said softly, more a question than a demand. An agent told her to stand aside and wait.

Harbury asked how long, and the officers said it could be several hours, perhaps days. She sat on the pavement with Gonzalez and the boy. "We'll wait," she said.

The officers appeared to notice a reporter taking notes and called a supervisor. He arrived and waved everyone through.

Gonzalez reached the inspection booth and pushed her paperwork forward. Harbury gave her a hug and an invitation to dinner. Then the officers directed Gonzalez and her son to an adjacent waiting room.

"They made it," Harbury said.

She waved goodbye through the glass. The room wasn't full, not even close. There were more than 60 chairs in the waiting area, and all but two were empty.

Excerpted from an article that originally appeared in The Washington Post. Reprinted with permission.