Carlos Hernandez Vásquez survived a journey of more than 1,000 miles from his home in Guatemala to the U.S. without his parents. Then, the 16-year-old was captured by U.S. Border Patrol on May 13, and a week later he died in U.S. custody. He was found dead on Monday morning, the agency said.

While Hernandez Vásquez is the fifth child to die after being apprehended by Border Patrol since December, his death is the first among those cases to draw the attention of federal law enforcement. The FBI is among the agencies investigating.

The boy crossed the U.S.-Mexico border near Hidalgo, Texas, on May 13, and was brought to a processing facility in the nearby city of McAllen, according to a Border Patrol official who briefed members of the press on Monday. There, adults and children are housed together in chain-link fence enclosures and given mats to sleep on.

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By law, unaccompanied migrant children in Department of Homeland Security (DHS) custody can be kept in such facilities for up to 72 hours before they must be turned over to the Office of Refugee Resettlement (ORR), which oversees shelters specializing in migrant child care. But three days after Hernandez Vasquez was brought to the McAllen facility, the two federal agencies had only just started processing his transfer, according to the Border Patrol official.

Carlos Hernandez Vásquez CBS News

Mark Weber, a spokesperson for the U.S. Department of Health and Human Services, which operates ORR, said there are rare exceptions to the 72-hour rule. He said a minority of cases exceed 72 hours and that such cases "have generally involved exceptional circumstances, such as health issues unique to the (child), or the placement of a sibling group to ensure they remain together."

In Hernandez Vásquez's case, another three days passed after authorities started processing his transfer, according to the official, who said the boy was given regular wellness checks during his time in custody. But officials have not said if they determined the boy was ill before he said he wasn't feeling well on Sunday, which was the day before he died, and six days into his time in custody.

ORR may have been better positioned than DHS to provide care for the boy, according to Peter Schey, the lead attorney for a team that oversees the Flores Agreement, a court settlement that governs the treatment of migrant minors in federal custody.

"ORR is in a far better position to conduct a more comprehensive medical assessment and to provide needed medical attention for detained minors," Schey told CBS News Tuesday.

A nurse concluded on Sunday that Hernandez Vásquez had the flu, and he was prescribed the drug Theraflu, according to the official. At that point, authorities wanted to separate him from other migrants who were living and sleeping in close quarters. He was driven to a Border Patrol station in Weslaco, Texas, about 20 minutes away, the official said.

By the time Hernandez Vásquez arrived in Weslaco, ORR had determined he would fly to the largest ORR shelter in the country, in Homestead, Florida, but then decided on another large, but much closer facility — Southwest Key Casa Padre in Brownsville, Texas. The Border Patrol official said the change was made because authorities did not want to make the sick teenager fly to another state.

It is not clear what treatment the boy received in the Weslaco station overnight. On Monday morning someone performed a wellness check, and it's unclear if any red flags were observed. An hour later, another wellness check was performed, and this time it was determined the boy was unresponsive, the official said. Medical officials could not resuscitate him.

News of Hernandez Vásquez's death sparked immediate recriminations from advocates and public officials.

"The gross inadequacy of the so-called 'welfare checks' is patently obvious," said Sen. Jeff Merkley, D-Oregon, a frequent critic of the federal government's immigration enforcement agencies. "I want to see specific plans from the administration to provide real medical checks, including checking vitals, on every child who comes into our government's custody. We need urgency, and we need answers, not excuses."

In a statement to CBS News, the Guatemalan consulate called for an investigation into the boy's death.

"The Guatemalan Government regrets the death of this Guatemalan boy, presents his condolences to the family and urges that the US authorities urgently rule on the cause of death and deduct the responsibilities that merit the case," the consulate said.

The Department of Homeland Security Inspector General is required to investigate all deaths in the agency's custody, and is taking the lead in this case, an FBI official told CBS News. The FBI and local police are also involved in the investigation.