The Department of Homeland Security (DHS) is holding unaccompanied children crossing the border without their parents in law enforcement custody for longer than three days on average—with some being held much longer—in violation of federal law requiring transfers to the Office of Refugee Resettlement (ORR) within 72 hours. ORR says it’s running out of space to house them all.

In 2019, DHS dealt with a similar rush of unaccompanied alien children (UACs) crossing without their parents, but this time is different. While large in absolute terms in 2019, UACs constituted just one in five Central American children crossing the border. Nearly 80 percent were crossing with their parents. Last month, about 70 percent crossed without either parent.

Something other than broad patterns of migration are causing kids to cross alone. One major reason for the switch started on September 29, 2019 when DHS decided to expel whole families under the Migrant Protection Protocols (MPP), otherwise known as Remain-in-Mexico, to await asylum hearings north of the border. Remain-in-Mexico was supposed to deter asylum seekers from applying. To some extent, it did deter many from trying. But it had another effect as well.

Once the families realized that would face months or years of homelessness in very dangerous cities in Mexico, many parents and their kids decided to separate. The Intercept’s Debbie Nathan was among the first to report on this terrible phenomenon in October 2019. Under MPP, families could be returned, but not UACs, meaning that parents to choose whether to separate from their children or put them at serious risk.

“I’m afraid to be here,” one woman said after explaining she would send her child. “I saw a woman and a child kidnapped.” After interviewing MPP families in Matamoros, Mexico in November 2019, NPR’s John Burnett reported that those stuck there even gave the decision to send their kids a name: la separación. Buzzfeed’s Adolfo Flores described how “dozens” of families were separating under the new policy.

The share of children crossing without either parent exploded in the months after MPP, reaching 69 percent in March. This was a higher percentage than any other month, including under the formal family separation policy in May 2018 (Figure 1). In March, DHS stopped using MPP to remove asylum seekers and switched to “expelling” them under a Centers for Disease Control (CDC) declaration (using authority under Title 42 of the U.S. code) that undocumented crossers posed a health risk, which the Trump White House ordered the CDC to issue over the objections of health experts.

Initially, the Trump administration applied the expulsion policy to all UACs traveling with any other adult family, which may have initially damped the flow in absolute terms (though because families were subject to the harsher policy the relative numbers remained mostly unchanged). But on November 18, a court ordered DHS to stop expelling any UACs, though DHS violated the order dozens of times through December 12. When Biden took over, he made the UAC exemption permanent on January 30.

But at the same time in January, the Mexican government in one state bordering south Texas started to refuse to accept families with young children back under Title 42 expulsion because a Mexican law that went into effect in November forbade detaining young children, so for the first time in months, it was possible to cross and obtain release for at least some families. About 60 percent of the families who crossed in February were not expelled, and as a result, both the share and absolute number of kids crossing with their parents jumped.

Nonetheless, the separations continue for older teens. The Washington Post’s Arelis Hernández interviewed a woman in February whose teenage daughters had crossed in January who said she had sent them “hoping that the new president would create a faster, legal route to get her case in front of a U.S. immigration judge.” She said that the separation was killing her. “I consider throwing myself in [the river] to die because the longer I’m here, the more I feel like I don’t want to live anymore,” she said. Not surprisingly, nearly two thirds of the unaccompanied children are 16- and 17-year-olds not subject to the Mexican law protecting young kids.

This family separation isn’t the only kind that the Biden administration is continuing either. DHS continues to classify children who cross with adult family members as “unaccompanied” in order to expel the family member to Mexico. This leaves the child stuck in ORR and CBP’s overcrowded facilities alone, and as the organization Kids In Need of Defense explains, both types of family separation make it far more difficult for the child to win an asylum case where evidence and witnesses are essential.

Many children crossing the border are truly alone and need ORR’s services to prevent them from becoming homeless, but most are crossing alone for reasons that Biden could easily correct. He should end Title 42 expulsions. He should stop separating kids from adult family members in custody. He should start admitting UACs at ports of entry.

Biden should also provide Temporary Protected Status or parole status to as many undocumented immigrants as possible so that they can go to the border without fear to meet their family members crossing at those border checkpoints. These actions would effectively solve the UAC border crisis.