An 8-year-old Guatemalan boy died of unknown causes in the custody of U.S. Customs and Border Protection minutes before midnight on Christmas Eve in New Mexico, the agency said in a statement Tuesday.

A Border Patrol agent noticed that the child, who was apprehended with his father, "showed signs of potential illness" on Monday, CBP said, citing initial reporting.

The father and son were then taken to a medical center in Alamogordo, New Mexico, where the child was initially diagnosed with a common cold, according to the statement. When the boy was evaluated for release, hospital staff found a fever and he was held for an additional 90 minutes before being released on Monday afternoon, CBP said. The boy was given prescriptions for amoxicillin, an antibiotic, and the painkiller ibuprofen.

On Monday night, the boy was nauseous and began vomiting and was sent back to the same hospital but lost consciousness en route. Hospital staff were unable to resuscitate the boy and pronounced him dead at 11:48 p.m. on Christmas Eve, the agency said in an updated statement late Tuesday. CBP had said previously that the boy died early on Christmas Day.

“This is a tragic loss. On behalf of U.S. Customs and Border Protection, our deepest sympathies go out to the family,” said commissioner Kevin McAleenan in the statement.

The boy's father was being detained late Tuesday at the Alamogordo Station pending transfer to Immigration and Customs Enforcement, CBP said in its updated statement. He has spoken to his spouse in Guatemala and the Guatemalan Consulate, the agency said.

Rep. Joaquin Castro, D-Tex., the incoming chairman of the Congressional Hispanic Caucus, identified the boy as Felipe Alonzo-Gomez in a statement Tuesday.

.@HispanicCaucus Chair Elect @JoaquinCastrotx: "I’m deeply saddened by the death of 8-year-old Felipe Alonzo-Gomez last night in @CBP custody and offer my condolences to his family." Full statement here: https://t.co/e0GWKQgPTJ pic.twitter.com/uRUFl9AuWa — Hispanic Caucus (@HispanicCaucus) December 25, 2018

Castro credited CBP for its timely public notification, but he also blamed the Trump administration's broader immigration policy for putting migrants in harm's way.

"The Administration’s policy of turning people away from legal ports of entry, otherwise known as metering, is putting families and children in great danger," Castro said in a statement.

The official cause of the child's death is still unknown and CBP's Office of Professional Responsibility will conduct a review, consistent with policy, according to the statement.

The hospital where the boy died, the Gerald Champion Regional Medical Center, said privacy regulations prevented it form sharing information about any individual patient.

"Our thoughts and prayers are with this family during this very difficult time," the medical center said.

The boy's death comes after Jakelin Caal Maquin, 7, also of Guatemala, died in Border Patrol custody earlier this month. Her body arrived in Guatemala on Monday.

Her death sparked outrage and calls for an independent investigation.

CBP announced last week that it would begin informing Congress and the media within 24 hours after someone dies in its custody.

The American Civil Liberties Union said in a tweet Tuesday that the boy's death was "a horrific tragedy" and that CBP should be held accountable. The group called for the incoming Congress to investigate the Department of Homeland Security, which includes CBP.

Rep. Pramila Jayapal, D-Wash., called the boy's death "deplorable."

"What happened to 'One death is too many,' @SecNielsen? " she said, quoting Homeland Security Secretary Kirstjen Nielsen's testimony before the House Judiciary Committee last week following Jakelin's death.

CORRECTION (Dec. 26, 2018, 3:09 p.m. ET): An earlier version of this article misstated the first name of the Guatemalan boy who died while in the custody of the U.S. Customs and Border Protection. He is Felipe Alonzo-Gomez, not Felix.