The number of migrants detained along the Mexico border crossed a new threshold last month, exceeding 200,000 for the first time in 21 years, according to U.S. Customs and Border Protection enforcement data released Thursday.

Among the 212,672 migrants taken into U.S. custody in July were 82,966 family members and 18,962 unaccompanied teenagers and children – an all-time high. The unaccompanied minors’ custody requirements have once more overwhelmed the Biden administration as it struggles to care for them safely in the middle of the pandemic.

Biden officials predicted earlier this year that the volume of people crossing the border would decline with the summer heat. Instead, Central American adults and children are crossing again in large groups of 300 or more, and U.S. border facilities are jammed with migrants shoulder-to-shoulder in detention facilities.

More than 15,000 minors who arrived without parents are in government custody, many sleeping in grim military barracks. A South Texas park along the Rio Grande has been converted into a sprawling quarantine camp for more than 1,000 parents and children who have tested positive for the coronavirus or been exposed to infection.

“The situation at the border is one of the toughest challenges we face,” Homeland Security Secretary Alejandro Mayorkas said Thursday, speaking in the Rio Grande Valley of South Texas, where the strain has been most acute. “It is complicated, changing and involves vulnerable people at a time of a global pandemic.”

The July arrest total marked a 13% increase from June, and was the second-highest number of arrests along the Mexico border on record, according to CBP statistics. Biden officials are projecting a record-setting year along the Mexico border, where authorities have stopped more than 1.3 million migrants since October.

The Biden administration continues to rely on a U.S. public health code known as Title 42 to bypass normal immigration proceedings and rapidly “expel” most single adult migrants to Mexico, but many attempt to cross again and again until they successfully evade capture.

The monthly data compiled by CBP is a gauge of enforcement activity along the border, rather than a count of distinct individuals. Of the more than 212,000 taken into U.S. custody last month, about 154,288 were unique individuals, Mayorkas said. CBP figures show 27% of border-crossers who were taken into custody in July had been previously detained.

In recent months, border officials have also reported more than 1,000 daily “got-away” incidents, in which authorities are able to detect an illegal entry – often using cameras and sensors – but do not make an arrest.

With the number of migrants arriving to the Rio Grande Valley of South Texas exceeding 20,000 per week last month, the Biden administration has introduced new enforcement measures in an attempt to deter illegal crossings. It has restarted fast-track deportation flights for some families who do not claim a fear of persecution if returned, while transporting others to border areas further west where Mexican authorities will accept the returns.

Most notably, Biden officials for the first time have launched “expulsion flights” that use Title 42 authority to fly Central American families deep into southern Mexico, hoping they will opt to return home rather than trying to reenter the United States. Hundreds of those migrants, including small children, have been dropped off in remote towns with little shelter capacity, rights advocates say, a pattern the United Nations refugee agency called “troubling.”

“At a time of significantly increased movement of asylum-seekers and migrants in the region, the Title 42 expulsion flights will also further strain the overburdened humanitarian response capacity in southern Mexico, heighten the risk of COVID-19 transmission across national borders and run counter to steps being taken to share responsibility among countries of the region in addressing the root causes of forced displacement and migration,” said Matthew Reynolds, the agency’s U.S. representative, in a statement.

Mayorkas addressed the flights to Mexico’s interior in his remarks, the first time U.S. officials have spoken publicly about the controversial practice.

“We are expelling them further into the interior of Mexico, where it’s far more difficult to try again,” he said. “We are working with Mexico to ensure for individuals subject to the expulsion flights [that] their needs are addressed.”

Mayorkas said the United States’ primary responsibility, however, is to “protect the American public” from the pandemic. The positivity rate of migrants tested along the Mexico border is on par with or lower than U.S. community rates, he added.

About 45% of border-crossers who arrived last month were turned back to Mexico, data shows, the lowest percentage since Joe Biden took office.

Along the U.S. southern border, U.S. Immigration and Customs Enforcement has sent 350 officers and staff to help CBP cope with the influx, senior ICE official Corey Price told agency employees Wednesday in an email obtained by The Washington Post.

Price also told staff that ICE’s Port Isabel facility in South Texas is being used as a large “staging site” where ICE personnel will process migrants CBP is too overwhelmed to handle.

Price also said in the email that ICE medical personnel have begun providing coronavirus vaccinations to some migrants in CBP custody. ICE officials, however, provided a statement late Thursday, attributed to Price, saying the agency is not vaccinating migrants in CBP custody.

Price’s email also told staffers that ICE was seeking more emergency bed space and staging facilities for the growing number of families arriving along the border.

Over the past several months, the arrival of soaring numbers of migrants from Ecuador, Brazil, Haiti, Venezuela and nations in Africa has complicated the Biden administration’s attempt to ease pressure on the border through a strategy based on addressing the “root causes” of emigration from Central America.

Officials in Panama this week held talks with Colombia and other nations in the region to urge tighter visa restrictions, as jungle camps holding U.S.-bound migrants have swelled with thousands of stranded migrants. About 10,000 people, mainly from Haiti and Cuba, are stuck in the small port town of Necoclí, Colombia, awaiting sea passage to Panama.