TEXAS, USA (AP) — A 16-year-old from Guatemala died of complications of the flu while in US Border Patrol custody, according to preliminary autopsy findings, alarming doctors who questioned whether immigration authorities missed warning signs or chances to save his life.

Carlos Hernandez Vasquez contracted bacterial infections in addition to the flu, as well as sepsis, which can lead to tissue damage and organ failure, according to a report released by Hidalgo County authorities this week. He died May 20. A full autopsy is pending.

Carlos is the sixth child in the last year to die after US border agents detained him, and the second known to have died of the flu, after 8-year-old Felipe Gómez Alonzo died on Christmas Eve.

As border apprehensions of migrant families surge to historic highs, the Trump administration’s treatment of migrant children and the detention of families in overcrowded facilities has come under harsh criticism. The Administration is asking Congress for $4.5 billion to respond to the surge of families, but Congress has yet to act.

Flu deaths among children are rare in the US. The number of annual flu-related deaths in kids has bounced from 37 to 186 since the 2004-2005 flu season, according to the Centers for Disease Control and Prevention. Doctors usually expect the youngest children, not teenagers, to be more at risk of severe complications and death from flu.

“Flu is uncommon at this time of the year,” said Dr Dawn Nolt, an infectious diseases expert at Doernbecher Children’s Hospital in Portland, Oregon. “That someone died of flu is certainly something to be very worried about.”

Two doctors said they were alarmed by what’s known about the Border Patrol’s response to Carlos’ illness.