A 10-year-old girl from Guatemala who was in custody of Mexican immigration officials died on Wednesday night after complaining about a sore throat and after seeing a doctor, officials said. 

Officials said the girl and her mother were taken by bus from the northern state of Chihuahua to Mexico City — a ride of at least 17 hours, according to a statement from the country’s National Migration Institute. They arrived at the Iztapalapa migration station on Tuesday, and the girl was seen by a doctor and taken to a pediatric hospital.

“The migration institute deeply regrets this event,” the statement said. Officials are investigating what may have led to the girl’s death. 

Mexico’s immigration agency has been overwhelmed as they've detained a growing number of U.S.-bound migrants in the southern part of Mexico and near the U.S. border. Some 13,000 are waiting to apply for asylum in border cities, according to an Associated Press report. More than 5,000 applicants have been returned to Mexico to wait for a court hearing under a controversial new Trump administration policy.

“The immigration agency lacks protocols for tending to migrant populations, and especially for populations presenting some type of illness,” said Jorge Andrade of the independent non-profit Institute for Democracy and Safety, which monitors the detention of migrants in Mexico.

Records obtained by students at the University of Texas at Austin LBJ School of Public Affairs show at least three minors died in custody of Mexican immigration officials in 2018.

Meanwhile, on Tuesday, a two-year-old Guatemalan boy died in Texas after being detained by U.S. Border Patrol agents.