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Fewer unaccompanied minors and families entered U.S. custody along the southern border in April than in March, but the number of overall apprehensions of migrants rose slightly, according to government data released Tuesday. The rise was driven by an increase in single adults, most of whom are being expelled to Mexico.

Customs and Border Protection (CBP) officials at the U.S.-Mexico border made more than 178,000 apprehensions in April, a slight increase from the 173,000 in March. More than 111,000 of those taken into custody were single adults. That demographic has been making up the bulk of recent border apprehensions, which have reached levels not seen in two decades.

Nearly 17,200 unaccompanied migrant minors and just over 50,000 parents and children traveling as families entered U.S. border custody in April, both down slightly from March, which saw record arrivals of lone youths.

“CBP continues to see a large influx of illegal migration along the Southwest Border,” the agency’s interim commissioner, Troy Miller, said in a statement.

About 111,000 of the 178,000 migrant encounters in April turned into rapid expulsions under the Title 42 public health authority, which was first invoked by the Trump administration in March 2020. Over 84% of single adults processed in April were expelled to Mexico or their home countries, the CBP data indicates.

Under the Title 42 process, which was authorized by the Centers for Disease Control and Prevention (CDC), migrants who don’t have permission to enter the U.S. are swiftly expelled to Mexico or to their home countries without a chance to apply for asylum. The Biden administration has relied on the Trump-era justification for the policy, saying the practice protects agents, migrants and border communities from the coronavirus.

“The use of Title 42 is driven by the public health imperative. It is not a tool of immigration. It is a tool of public health to protect not only the American people but the migrants themselves,” Homeland Security Secretary Alejandro Mayorkas told reporters during a visit to Donna, Texas, on Friday.

The Biden administration has not been expelling unaccompanied children under the Title 42 policy, a practice that was carried out under former President Trump until a federal court blocked it last November. While it has said that migrant families with children continue to be subject to the Title 42 emergency order, the Biden administration has been expelling a smaller percentage of the families it has encountered in recent months.

In April, 34% of migrant parents and children taken into custody as families were expelled to Mexico or to their home countries. The rest were processed under U.S. immigration laws and allowed to stay in the country while they seek forms of humanitarian protection like asylum.

In March, when a monthly record-high 18,800 unaccompanied children entered U.S. custody, the Biden administration found itself facing significant humanitarian and logistical challenges at the southern border. Border Patrol facilities, which are not designed to hold migrants beyond 72 hours, were overcrowded and packed with thousands of unaccompanied children.

However, the number of unaccompanied minors in Border Patrol custody has dropped by 92% since reaching a record high 5,800 in late March, falling below 500 on Tuesday morning, according to government data. Overcrowding at the largest Border Patrol holding facility for migrant children had also been dramatically reduced.

The main reason for the change in conditions is that the Department of Health and Human Services, which is charged with the long-term care of unaccompanied children, has converted more than a dozen convention centers, work camps, military sites and other facilities into makeshift shelters for the minors.

As of Tuesday morning, HHS’ Office of Refugee Resettlement was housing nearly 22,700 unaccompanied children, many of them at the emergency sites. The refugee agency is tasked with placing these minors with vetted sponsors, who are typically relatives living in the U.S.