Dallas — A 16-year-old boy from Guatemala who survived the thousand-mile journey to the U.S. border died Monday in U.S. custody.

The Congressional Hispanic Caucus is calling for a federal investigation after the fifth migrant child since December died after arriving at the U.S. border.

"This is an epidemic of death. Nobody had died for 10 years. And in the last six months, you've had five deaths," Rep. Joaquin Castro said.

Carlos Hernandez Vásquez CBS News

The latest, 16-year-old Carlos Hernandez Vásquez of Guatemala, was an unaccompanied minor. The teen was apprehended at the Texas-Mexico border on May 13.

On May 19, he told the medical staff he didn't feel well and was given Tamiflu. He was found dead the next day.

By law, immigration officials have three days to transfer unaccompanied minors to the Office of Refugee Resettlement (ORR) — a special division that deals with children.

Carlos Hernandez's brother, blurred to conceal his identity. CBS News

CBS News spoke to one of Hernandez's brothers, in New Jersey, who just wants to know what happened.

"The hardest part about it all is what happened to him because we never thought this would happen in a place where he's supposed to be in a better place," he told CBS News through a translation.

An official with the Guatemalan Consulate in New York told CBS News late Tuesday that Carlos was traveling with his sister. They were separated when apprehended by U.S. Customs and Border Protection because she's a 21 year-old woman and Carlos was a 16 year-old.

As a result of Carlos' death, the consulate asked CBP to release his sister, and the agency did. She was on her way to join her bothers in New Jersey.

The five Central American boys and girls who have died since December ranged in age from 2-1/2 to 16.

CBS News

Four died in Border Patrol facilities, while the youngest died in an El Paso hospital after being released from government custody.

Acting Homeland Security Secretary Kevin McAleenan addressed the overcrowded southern Border Patrol facilities on Sunday's "Face the Nation."

"I'm very concerned about the conditions," he told moderator Margaret Brennan. "These are not appropriate facilities for families and children in particular. These are police stations built for single adults."

To ease the overcrowding, the Department of Homeland Security is now flying migrants to other cities across the U.S., just to start the immigration process.

CBS News was the only broadcast network on the tarmac in Brownsville, Texas, as Border Patrol and ICE officials loaded three busloads of migrants onto a jet.

Some family units could be seen: moms, dads and younger kids. From here, government officials check their pockets to see if the migrants have anything in them. They are then loaded onto an airplane and from there that plane will take off and transport them to a destination where they will be further processed. The migrants don't know where they are going.

Some migrants were taken on a short flight, roughly 400 miles, to Del Rio, Texas. The journey is expensive: each flight costs taxpayers between $20,000 to $60,000 each. ICE told CBS News that's about $9,000 an hour.

A United Nations monitor asked the Trump administration for a formal visit to U.S. Border Patrol facilities; they have ignored those requests.