In the the Rio Grande Valley in South Texas, the families keep coming.

On a recent day, hundreds of families passed through a welcome center run by a local nonprofit called Team Brownsville, where they picked up donations of food and clothing before continuing on their way north.

"We are starting again from scratch," said Francisco Sierra, who fled from Venezuela with his wife and two young sons. "We came here with nothing but our clothes."

The number of migrants crossing the U.S.-Mexico border was down for a few months. Now those numbers are climbing again, on pace to match the historic highs of last year.

That's driven in part by a record-breaking influx of migrant families. Immigration authorities arrested more families in August than any month on record: more than 90,000 people altogether border-wide.

The scale might be new, but immigration experts say the underlying issues are not. Three administrations in a row have grappled with how to discourage migrant families from crossing the border illegally — and found that there are no easy solutions.

"We have not seen any policy reduce arrivals in the long run," said Theresa Cardinal Brown is a former Homeland Security official who's now with the Bipartisan Policy Center in Washington.

Brown says immigration authorities are limited in how long they can hold migrant children in detention because of a long-standing legal agreement known as the Flores Settlement.

Still, the Obama administration did hold migrant families in special detention centers, hoping that would deter others from crossing. But Brown says it didn't work and migrant families kept coming.

"It's very hard to deter somebody who has that level of desperation through harsh penalties," she said, "especially migrants who believe that if they do not come to America their family will die or their kids will be killed."

But former President Trump's administration was determined to try, arguing that families and smugglers were taking advantage of generous U.S. policies. The Trump administration used even harsher tactics, deliberately separating children from their parents at the border.

"A big name of the game is deterrence," then-White House chief of staff John Kelly explained to NPR in 2018. Family separation "could be a tough deterrent, would be a tough deterrent."

Again, migrant families kept coming. And the Trump administration was forced to abandon family separation amid widespread blowback.

President Biden promised a more humane approach at the border. His administration has decided not to bring back family detention. It's focusing instead on alternatives — including ankle monitors and curfews for migrant families that are placed in a program known Family Expedited Removal Management, or FERM.

The Biden administration said this week that it's expanding the program, which is intended to speed up immigration proceedings for migrant families. So far, the administration has processed only 1,600 families since launching the FERM program in May.

The White House is also asking Congress for permission to reprogram some funding for "community-based residential facilities" for migrants.

"It seems like family detention, but painted with a different brush," said Cindy Woods of Americans for Immigrant Justice. "It's another attempt to deter more families from coming to the United States."

Woods and other advocates argue that this effort won't work either, because these families are so desperate.

Before leaving Venezuela with his family, Francisco Sierra worked as a professor of education. His wife was an engineer in a chemical plant. Still, Sierra says they were barely getting by due to Venezuela's crumbling economy, and saw little hope for their two boys, ages 4 and 5.

"The effort is no longer worth it," Sierra said in Spanish. "My career was practically six wasted years studying at a university. ... So we looked for a way to emigrate to have a better future for the family."

Even Homeland Security officials can seem a bit shocked by the desperation of migrants who are willing to cross the dangerous Darién Gap, a remote border crossing through the jungle in Panama.

"It is heartbreaking," Blas Nuñez-Neto, a top immigration official at the Department of Homeland Security, said during a conference in Washington this week. Nuñez-Neto recently traveled to the Darién Gap, where tens of thousands of migrants per month are making their way north toward the U.S.

"You see families with really small children, babies, kids in diapers. Coming out of that jungle after having walked for four or five days with no food and little water. Just in really dire conditions," he said.

Even the perils of the jungle aren't enough to stop these families from coming.

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