The Biden administration's attention along the Mexico border has been consumed for the past several weeks by the record numbers of migrant teenagers and children crossing into the United States without their parents, at a rate that far exceeds the government's ability to care for them. 

But as they race to add shelter capacity for these minors, Department of Homeland Security officials are privately warning about what they see as the next phase of a migration surge that could be the largest in two decades, driven by a much greater number of families.

DHS expects roughly 500,000 to 800,000 migrants to arrive as part of a family group during the 2021 fiscal year that ends in September, a quantity that would equal or exceed the record numbers who entered in 2019, according to government data reviewed by The Washington Post. Officials are now racing to find facilities to house these families ahead of their release, along with additional staff to process an increase in humanitarian and asylum claims.

The estimate is based on what has already been a vertiginous increase since President Joe Biden took office Jan. 20. This month, the number of family members taken into Customs and Border Protection custody is on pace to reach nearly 50,000, up from 7,000 in January, the latest government data show. The highest one-month total, 88,587, was recorded in May 2019, during a year when more than 525,000 migrants arrived as part of a family group.

Groups of families - sometimes collectively numbering as many as 400 - have been showing up this month along the riverbanks in South Texas, straining CBP's ability to transport, process and care for so many parents and children without leaving other sections of the border unsupervised.

Hundreds of parents and children have been spending hours at an outdoor processing station next to the Rio Grande, some sleeping on the ground while they wait for agents to formally take them into custody.

Roy Villareal, who retired last year after 33 years in the Border Patrol, said roughly 40 percent of those taken into CBP custody now are children and families, but they consume 60 to 70 percent of agents' time, attention and paperwork.

"Border security drops tremendously because of this," he said, noting that drug traffickers often choreograph the crossings of large groups of families to tie up agents in one area while moving narcotics in another.

While the Biden administration says its policy is to "expel" families to Mexico under a pandemic health order, the most recent CBP data shows that only about 10 to 20 percent are being turned back. The rest are typically released into the United States with a notice to appear in court, even though Biden told reporters last week the families "should all be going back."

In late January, just days into Biden's term, Mexican authorities stopped accepting some families rejected by U.S. agents, primarily in the Rio Grande Valley of South Texas, citing a new child protection law that has limited their shelter capacity. The rule has applied mostly to families arriving with children under the age of 7, so parents with small children have rushed to that span of the border over the past two months, hoping to be quickly released into the United States.

The Biden administration has placed some families arriving to South Texas on flights to other sectors of the border, including El Paso, then returning them to Mexico from there. But there appears to be no formal determination as to who is allowed into the United States and who is selected for the expulsion, sowing confusion and anguish among the families unlucky to be turned away.

"It is a little bit unknown as to what population gets transferred to El Paso," a U.S. Border Patrol official said Friday during a media briefing provided on the condition that the official not be identified.

Biden sent top diplomats to Mexico last week in an effort to get President Andrés Manuel López Obrador to accept the return of more expelled Central American families, following an agreement that will send millions of surplus AstraZeneca coronavirus vaccine doses to Mexico from U.S. stockpiles.

There has been no formal announcement about expanded Mexican shelter capacity, however, and the extraordinary volume of people arriving to South Texas has often left U.S. agents too overwhelmed to complete the paperwork. They have started handing some families blank forms and asking them to report to U.S. Immigration and Customs Enforcement later, a practice CBP officials say they have never engaged in on a large scale.

"Our policy remains that families are expelled, and in situations where expulsion is not possible due to Mexico's inability to receive the families, they are placed into removal proceedings," said Sarah Peck, a DHS spokeswoman, referring to the deportation process.

Though family groups do not require the same intense level of care as unaccompanied minors, they often require short-term shelter stays after they are freed, as well as transportation to their destination in the United States. During the last major influx of families in 2019, nonprofit groups worked with federal authorities to house families in hotels, tents and converted warehouses, a sprawling humanitarian relief effort that will probably need to be ramped up again.

While the Biden administration is expelling some families, it is allowing nearly all unaccompanied minors to stay, so some parents are choosing to send their sons and daughters across the border alone. As more families are allowed to stay, more parents are expected to arrive with their children, instead of splitting up.

Cris Ramón, an immigration consultant in Washington, said Mexican immigration data last year began showing a new migration wave was building, "not unlike the way you see weather developing." But neither the United States nor Mexico appears to have prepared.

"The moment your border system collapses is not the time to be looking at data and saying we need a response," Ramón said. "It's just too late. You need to look at fundamentals in regional data and start planning two to three months out."

After the Obama administration faced a spike in family migration in 2014, it opened large "residential centers" for parents and children that were designed to resemble dormitories rather than detention facilities. But courts have limited the amount of time minors are allowed to spend at those sites, and the government has struggled for years to rule quickly enough on their claims for humanitarian protection.

That system was quickly overwhelmed during the 2019 surge, and so many families arrived at once that the Trump administration had to quickly free them into the United States, a practice it derided as "catch and release." Their numbers fell again after President Donald Trump expanded the "Remain in Mexico" program, which required families seeking asylum to wait outside U.S. territory, often in grim camps and dangerous border cities where they were vulnerable to kidnappers and extortionists.

Immigration and Customs Enforcement announced this month that it will expand its capacity to hold families near the U.S.-Mexico border to more than 3,700 beds in coming weeks, including 2,500 beds at a pair of existing family residential centers in South Texas, records show. ICE has also converted its two largest family detention sites into rapid-processing hubs to facilitate the release of parents with children within 72 hours.

Fewer than 500 family members were in custody in early March, but their numbers have soared as high as 1,200 in recent days.

Starting in early April, ICE will hold more than 1,200 family members in hotel rooms under a new nearly $87 million contract with a nonprofit called Endeavors. ICE plans to release the families from the hotels within 72 hours, after providing them health screenings, a coronavirus test and access to clothing, meals, snacks and unlimited phone calls. ICE also will coordinate with nonprofits to find them shelter, food and transportation once they are released.

Officials cautioned that most people apprehended at the border are still being expelled under the Centers for Disease Control and Prevention's March 2020 order under Title 42 of the public health code.

But that did not apply to most families in February, and the percentage expelled has fallen even lower in March. In an email, a Justice Department lawyer told advocates for child migrants that it planned to use hotels as staging areas for families before releasing them, because of "the exigent circumstances currently occurring along the southwest border."

"The expectation is that release will occur as expeditiously as possible and, except in very rare cases, custody will last no longer than 72 hours," Justice Department lawyer Sarah Fabian told lawyers in a March 6 email, according to court records.

One veteran former Homeland Security official said plans to use hotels would rapidly exhaust capacity and budgets, creating "a massive operational challenge."

"If they have 1,500 hotel beds, that's half a day's arrival numbers by time they're up and running in April," said the former official, who spoke on the condition of anonymity because the former official did not want to be quoted criticizing the new administration. "You'll have situations where local communities are overwhelmed at their own shelters and at transportation centers leaving the border."

Villareal, who was Border Patrol chief in Arizona's busy Tucson sector, said agents are especially concerned that the practice of releasing families with blank paperwork and without a notice to appear in court will generate more illegal crossings.

He said the Biden administration is right to look at the root causes driving people to leave Central America, but cautioned that a more "holistic approach" is needed.

"You need an enforcement aspect, and you need an investment aspect," Villareal said. "We have to recognize this is one large system, and piecemeal efforts only undermine one aspect."

"If you're securing and investing in Central America, and giving people the ability to apply for asylum in their home countries with increased immigrant visas, there is a long-term benefit to be had, because not only will it help secure the border, it'll better formalize legal migration," he added. "But if you want to further legal migration, you just can't do that part and parcel. You have to maintain border security to have more humane and robust legal-migration framework."

The asylum claims and humanitarian appeals of families released from custody will add to what is already an immigration court system clogged by a backlog of more than 1 million cases. Of the roughly 1 million family members taken into custody by CBP between 2014 and early 2020, about 6 percent have been repatriated to their home countries or have a confirmed departure, and roughly 5 percent have been granted some form of legal status, according to DHS statistics.

The cases of the remaining 89 percent remain unresolved, including 67 percent whose claims are pending and 20 percent who have been ordered to leave the country but lack a confirmed departure, the figures show.