The Mexican government has stopped taking back Central American families “expelled” at the U.S. border under a Trump-era emergency health order related to the coronavirus, a shift that has prompted U.S. Customs and Border Protection to release more parents and children into the U.S. interior, according to five U.S. officials.

The change, which has not been publicly disclosed, raises concerns in U.S. border communities and at the Department of Homeland Security because the large-scale release of parents and children into the United States has triggered previous waves of unauthorized migration.

U.S. officials say the releases have occurred mostly in south Texas, including CBP’s Del Rio sector and the Rio Grande Sector that is the border’s busiest place for illegal crossings. In a statement, CBP spokesperson Stephanie Malin acknowledged an increase in the number of families released after being taken into CBP custody.

“CBP has seen a steady increase in border encounters since April 2020, which, aggravated by COVID-19 restrictions and social distancing guidelines, has caused some facilities to reach maximum, safe holding capacity,” Malin said in a statement.“Whenever feasible we are seeking alternatives to detention in cases where the law allows.”

Under an emergency order issued by the Centers for Disease Control and Prevention last March, U.S. authorities have carried out more than 390,000 “expulsions” along the Mexico border, allowing agents to bypass normal immigration proceedings and rapidly turn back roughly 90 percent of unlawful crossers. The measures are necessary to avoid transmission of the virus inside Border Patrol stations and immigration jails, CBP officials say.

U.S. authorities have made more than 70,000 detentions and arrests along the Mexico border for each of the past four months, one of the busiest periods of the past decade, according to the most recent CBP figures and projections. The daily arrest totals have increased over the past week, according to three U.S. officials, a trend driven by family groups and children.

“Mexico is only accepting single adults now, not families or children,” said one U.S. official who, like others, was not authorized to speak publicly about the change.

Mexico’s Foreign Ministry did not respond to requests for comment. But a new Mexican law approved in November mandates that children and families can no longer be held in immigration detention facilities, a reform applauded by human rights groups.

On the border, the policy meant families and children were sent to specific government family shelters instead. In some border cities, the shelters quickly filled up. Mexican immigration officials informed their U.S counterparts that, in those cities, families could not be returned to Mexico.

At the child welfare shelter in Reynosa, Mexico, just south of McAllen, Tex., officials said they have been at capacity since December. Normally, the limit is set at 50 people, but because of the pandemic, it has been reduced to 25.

“We just can’t accept any more people right now,” said Carlos Lozano, an official at the state child welfare agency who helps oversee the shelter.

He added that before the new law was implemented parents would often be separated from their teenagers, who would remain in the shelter alone. Now those families must be kept together, Lozano said, which has added to capacity issues.

Mexico’s new policy has been applied unevenly. In Nogales for example, families continued to be returned to Mexico — deposited on the streets rather than taken to family shelters. In a number of border cities, families continue to be returned to Mexico.

The number of migrants arriving as part of family groups per month peaked at more than 88,000 in May 2019, and then fell quickly after the Trump administration pressured Mexico to crack down and take back Central Americans ordered to wait outside U.S. territory. Between 4,000 and 5,000 migrants traveling in family groups have been taken into custody in recent months while CBP relied on the expulsion measures as its main enforcement tool.

The Biden administration directed CBP to stop expelling minors who arrive without a parent, allowing them to go to U.S. shelters instead, while launching a broader review of Trump’s deterrent policies and border controls, including the CDC order.

The United States has had far more coronavirus infections and deaths than any other nation. Mexico has had the world’s third-highest number of deaths, and President Andrés Manuel López Obrador announced last week that he had tested positive.

Emergency public health orders already restricted nonessential legal travel at U.S. land borders, and President Biden issued a Jan. 21 executive order that required visitors arriving international airports to show negative coronavirus test results before entry.

In the Rio Grande Valley of South Texas, adults arriving with children are now being taken to the Border Patrol station in McAllen for processing, and then typically released into the U.S. interior, border officials said. CBP officials do not administer coronavirus tests to migrants in their custody and do not have the capacity to do so.

Roel “Roy” Rodriguez, the city manager for McAllen, said he and other officials spoke to CBP several times last week to share their concern about the release of families without adequate testing. The pandemic has had a devastating impact in South Texas, with more than 100,000 infections in the Rio Grande Valley and one of the highest death rates in the state.

“We’re very concerned,” said Rodriguez. “We can handle a surge, we can handle immigrants, but the difference now obviously is covid.”

Rodriguez said Texas emergency management officials have sent testing kits to the Rio Grande Valley for use by the shelters that house migrants after they’re released by CBP. But he said families that leave the area after release would not necessarily be tested. Also unclear was how the shelters would handle those who test positive.

The largest shelter in McAllen has capacity for about 1,200 in normal conditions, but that has been reduced to 300 because of the pandemic, Rodriguez said.

One U.S. official working to address the situation said the Biden administration is working with Mexico to increase shelter capacity for families south of the border, while coordinating the release of families through U.S. Immigration and Customs Enforcement (ICE), which has better testing capacity and the ability to more carefully track families making asylum claims and seeking humanitarian protections.

Sister Norma Pimentel, the executive director of Catholic Charities of the Rio Grande Valley, said she met last week with senior Border Patrol officials, who told her to expect a surge in migrant families who would be released upon entering the United States. Since then, she has received 50 to 80 people a day.

“We got 100 people the first time, and since then it’s been group after group after group,” she said.

Sometimes the families come in crowded buses from as far as 50 miles away, at all hours of the day and into the night. Pimentel and her staff provide food, hygiene products and in some cases coronavirus tests to the migrants before they take Greyhound buses to different parts of the United States.

“I’m glad that after years they are able to be processed in the United States because it’s not safe for them in Mexico,” she said.

Last month a truck carrying Guatemalan migrants in northern Mexico was found scorched and plastered with bullet holes. Prosecutors said this week at least 12 police were involved in the massacre — underscoring the risks facing migrants near the border, including from state security forces.

A Central American official who closely monitors migration dynamics said smuggling guides have intensified their marketing efforts in Guatemala’s destitute rural highlands in recent weeks, recruiting customers by telling them the Biden administration is taking a softer enforcement approach.

“They’re saying Biden has given the green light,” said the official, who was not authorized to speak to reporters.

The Biden administration said it is working as quickly as possible to undo Trump policies it considers harmful and inhumane, replacing them with an approach that is more welcoming to immigrants and offers protections to vulnerable groups seeking safe refuge.

Biden officials say their message to migrants considering a journey is “this is not the time to come to the United States.”

“We need the time to put in place an immigration process so people can be treated humanely,” White House spokeswoman Jen Psaki told reporters this week.

Sieff reported from Mexico.