Eva Acuña spoke with her teenage sister Esther by phone early on the morning of Aug. 15, about an hour before Esther planned to enter the U.S. near the Mexican city of Ciudad Juárez and ask for asylum — the end of a long journey from Esther’s home in El Salvador.

Acuña, a legal permanent resident in the U.S., expected to hear next from U.S. immigration authorities about her sister’s status. But instead, about eight hours later, she received a call from authorities in Mexico. Instead of taking her sister into custody, the U.S. Border Patrol had delivered the girl back to Mexico, where she was in a children’s shelter.

The transfer was contrary to both U.S. policy and an outstanding diplomatic agreement with Mexico, which do not allow children from other countries who are traveling without adult guardians to be expelled into Mexico. But it is now becoming clear that a number of children have been improperly expelled after the Trump administration shut down the border to most asylum applicants because of the coronavirus pandemic.

Since the New York Times reported last week on an internal e-mail that warned border authorities about the improper transfers, Acuña, who asked that her sister be identified by her first name to avoid immigration repercussions, is one of several Central Americans who have come forward saying they were anxious and confused after their children and young relatives were sent without any adult to accompany them into a country that is not their own.

Sending young people back and forth between foreign governments is a sensitive matter, in part because of the bureaucratic red tape that can lead to delays in their release, even when the child’s parent is waiting in the same country.

It is unclear how many non-Mexican children have been expelled into Mexico, because both the U.S. and Mexican governments have declined to provide data. U.S. government officials have cited a legal challenge against some of the expulsions that have occurred under the pandemic to explain why they cannot elaborate further. In a tweet Friday, a spokesman for the Mexican secretary of foreign affairs said that “at the moment” it had no record of minors entering Mexico without accompanying relatives.

“The Mexican government, along with civil society and multilateral organizations, will continue with due investigations,” the statement said.

A U.S. Border Patrol official raised alarms about the practice in the internal e-mail that came to light last week. Brian Hastings, chief of the Border Patrol’s Rio Grande Valley sector, confirmed the practice had been occurring, and said border agents had been directed to contact the Mexican consular office each time an unaccompanied child who was not Mexican was expelled.

Five people have told the Times that their children or young relatives were expelled into Mexico after entering the U.S., in violation of the agreement between the two countries.