GUATEMALA CITY/NEW YORK — The 8-year-old Guatemalan boy who died in the custody of U.S. border agents this week had the flu before he passed away but the cause of his death is still unknown, state officials said on Friday.

Felipe Gomez Alonzo was the second Guatemalan child to die this month while being held by U.S. authorities, sparking outrage from immigration advocates.

In the wake of the deaths, Department of Homeland Security Secretary Kirstjen Nielsen was traveling to the border to observe medical screenings and conditions at Border Patrol stations while Democrats in the House of Representatives and Senate have called for a full investigation.

Gomez fell ill after being detained near the U.S. border with Mexico with his father. The two had traveled to the United States from their remote village of Yalambojoch in Guatemala.

New Mexico’s Medical Investigator’s office said in a statement that nasal and lung swabs during an autopsy found Gomez had influenza B, but said “determining an accurate cause of death requires further evaluation.”

U.S. Customs and Border Protection did not immediately respond to a request for comment on the flu diagnosis.

Marta Larra, a spokesman from the Guatemalan Foreign Ministry, said the government expected to receive the official results of the autopsy in roughly a month, and that Gomez’s body would probably be returned to the country in about two weeks.

Gomez and his father were detained on Dec. 18 in El Paso, Texas, for illegally entering the country, but several days later they were moved to the Alamogordo Border Patrol Station in New Mexico.

After a border agent noticed the boy looked sick, Gomez was transferred to a nearby hospital but he was released with a prescription for an antibiotic and Ibuprofen. After the boy began to vomit and had a fever, he was taken back to the hospital in the evening, where he died just before midnight on Dec. 24.

Antibiotics do not work as treatment for viruses like the flu, according to the U.S. Centers for Disease Control and Prevention.

Kathryn Hampton, who coordinates the Asylum Network Program of the New York-based advocacy organization Physicians for Human Rights, said in a statement that border facilities are not well equipped to handle the increasing number of children and families migrating to the United States.