Secretary of Homeland Security Kirstjen Nielsen on Wednesday called the death on Christmas Eve of an 8-year-old Guatemalan boy in federal detention a "deeply concerning and heartbreaking" tragedy and cited U.S. immigration system failings for a growing border crisis.

The top Trump administration border security official spoke out after directing a series of actions to care for undocumented immigrants taken into custody after illegal Mexican-U.S. crossings. The moves include U.S. Customs and Border Protection medical exams for all children it holds in custody.

The statement and actions marked the latest federal response to the death of the boy identified by Guatemalan officials as Felipe Gómez Alonzo. He was pronounced dead at 11:48 p.m. MST on Monday after he fell ill while held in CBP custody with his father, Agustín Gomez.

Felipe was the second immigrant child to die while in federal detention this month.

In a statement to the El Paso Times, BorderRAC Executive Director Wanda Helgesen said that as of Wednesday morning, 450 minors had been screened at regional medical facilities in the Border Patrol's El Paso sector, which includes New Mexico.

Details on the specific number of migrant patients at each facility were not available late Wednesday.

Nielsen's formal statement cited a recent jump in illegal border crossings by immigrant families and unaccompanied children. CBP apprehensions in those categories rose 86 percent – more than 68,000 family units and almost 14,000 unaccompanied children – over last year’s totals for the most recent two months along the Southwest border, she said.

"Our system has been pushed to a breaking point by those who seek open borders. Smugglers, traffickers and their own parents put these minors at risk by embarking on the dangerous and arduous journey north," Nielsen said.

"This crisis is exacerbated by the increase in persons who are entering our custody suffering from severe respiratory illnesses or exhibit some other illness upon apprehension," she said. "Given the remote locations of their illegal crossing and the lack of resources, it is even more difficult for our personnel to be first responders."

Nielsen said the changing immigration dynamic has been spurred by "an immigration system that rewards parents for sending their children across the border alone," without requiring the adults to face "consequences for their actions."

She cited an asylum process that’s unable to provide swift help for qualified applicants and an immigration system that encourages fraudulent claims by border crossers. She noted nine of 10 asylum applications get rejected by immigration judges.

Nielsen said she contacted Mexican border officials to seek an investigation into "the cause of these illnesses on their side of the border and to provide medical assistance in shelters as needed."

She planned a trip to CBP patrol stations this week to review medical screenings and conditions – conditions that have spurred questions and criticism among some Democrats.

The updated health testing directed by Nielsen marks a change in the handling of young children detained after entering the USA without required documentation and comes amid a partial shutdown of the federal government over President Donald Trump's demand for $5 billion to build a security wall at the nation's southern border with Mexico.

CBP said Tuesday night that it started secondary medical checks beyond initial border screening on all children in custody. More than 95 percent of those children had been re-examined by paramedics, emergency medical technicians or medical facilities near the border, the officials said Wednesday.

In a background phone briefing Wednesday, the officials said CBP is considering options from other federal agencies to help provide increased medical assistance for young immigrants at the border. Along with support from the U.S. Coast Guard, assistance could come from the Department of Defense, the Federal Emergency Management Agency and the Department of Health and Human Services.

The CBP update included a timetable of events leading to the latest death. It showed that the boy and his father had been in custody since Dec. 18.

A CBP agent noticed that the boy "was coughing and appeared to have glossy eyes" at approximately 9 a.m. MST on Monday. Taken with his father to a hospital in Alamogordo, New Mexico, he was diagnosed with a cold and a fever, prescribed amoxicillin and Ibuprofen and was released Monday afternoon after being held 90 minutes for observation, CBP said.

The boy was returned to the hospital Monday evening with nausea and vomiting and died hours later, CBP said. The hospital – the Gerald Champion Regional Medical Center – declined to comment Tuesday, citing privacy regulations.

The federal officials on Wednesday declined to discuss the hospital's earlier decision to release Felipe. Such decisions are left to medical professionals and are not made by CBP, they said.

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

The official cause of the boy's death had not immediately been determined, pending results of an autopsy. CBP's Office of Professional Responsibility and the Office of Inspector General will conduct a review, the officials said.

CBP asked the U.S. Centers for Disease Control and Prevention (CDC) to seek potential causes for a recent increase in the number of migrants in U.S. border areas who require medical aid. CBP personnel have transported "dozens" of people per day to hospitals for care in recent months, the officials said.

An official headcount on the number of immigrant children in CBP custody was not immediately available. The officials estimated Wednesday that families with children and unaccompanied immigrant children accounted for 1,400 to 1,500 of the roughly 2,100 people who are apprehended or detained at border areas each day.

Oscar Padilla, the Guatemalan consul in Phoenix, said Felipe's father told him the two had traveled to the USA from their home in Nentón, a village about 280 miles from Guatemala City. They planned to go to Johnson City, Tennessee. The consul identified the 47-year-old father and said he remains in U.S. Border Patrol custody.

The Border Patrol apprehended the father and son Dec. 18 for illegal entry, about 3 miles west of the Paso Del Norte Port of Entry in El Paso, Texas. Agents detained them at that port's processing center, logging six welfare checks until Thursday when they moved them to the El Paso Border Patrol Station.

There, the agency said it logged an additional 17 welfare checks before transferring them to Alamogordo Station on Sunday to finalize processing because of capacity levels at El Paso.

Agents conducted several welfare checks in Alamogordo before a processing agent noticed Felipe's symptoms at 9 a.m. local time Monday. At all three places where the father and son were detained, they were given food and drinks, as well as showers or personal hygiene products at the last two stations, CBP said.

Felipe's death represents an "incredibly rare" tragedy, the federal officials said Wednesday. Six people have died in CBP custody in 2018, Nielsen said.

More:7-year-old Guatemalan migrant girl in Border Patrol custody dies from high fever, seizures; feds fault father

More:Body of Jakelin Caal, who died in US Border Patrol custody, arrives in Guatemala

More:Girl who died in Border Patrol custody fled impoverished Guatemala village

The second death of a child this month reignited criticism of the federal government’s handling of migrant children and their families.

The incoming chair of the Congressional Hispanic Caucus, Rep. Joaquin Castro, D-Texas, offered his condolences to the family and called for an investigation.

"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. "With two deaths that we know about just in the last few weeks, Congress will continue to press the Department of Homeland Security until we get answers to all our questions.”

Alamogordo, where the boy died, is about 90 miles from the U.S.-Mexican border at El Paso. Ruben Garcia, director of El Paso’s Annunciation House, said Tuesday that he had no reason to believe his shelter had served the family but was waiting for further details about what happened.

A spokeswoman for U.S. Rep. Steve Pearce, R-N.M., whose district along the U.S.-Mexican border includes Alamogordo, could not be reached Tuesday.

Xochitl Torres Small, a Democrat who will represent the House district starting in January, called for a thorough and transparent investigation into the children’s deaths and medical resources along the border.

She said she had not personally inspected the holding area at the CBP checkpoint between Las Cruces and Alamogordo, where Felipe was held in between hospital visits. Nonetheless, Torres Small said, she worried that conditions in such facilities "could be exacerbating illnesses."

Alluding to overcrowding and hygiene, she said it is critical that CBP determine the best way to transport a sick child, how to ensure availability of medical personnel to give evaluations and how to allow people to take a shower.

The previous migrant child who died in government custody was Jakelin Caal, 7, a Guatemalan girl. She died Dec. 8 in El Paso after being apprehended by border agents.

Jakelin and her father crossed the border in New Mexico on Dec. 6, along with 161 other migrants. They were scheduled to travel by bus to the Lordsburg Border Patrol station in New Mexico when her father, Nery Gilberto Caal, told Border Patrol agents she was sick. She was transported to a children's hospital in El Paso where she died.

The White House said it had no responsibility in the "horrible, tragic" situation.

CBP, which did not notify Congress about Jakelin's death for days, said it would follow new procedures when someone dies in its custody. The protocol, including notification of federal lawmakers within 24 hours of any death, was followed after Felipe died, federal officials said Wednesday.

Rep. Lou Correa, D-Calif., expressed dismay over Tuesday's passing of the 8-year-old boy. In a tweet to Nielsen, Correa wrote, “This is the second child in a month. What is going on at @DHSgov? Does @HouseHomeland have to start subpoenaing you to get the truth?”

Other Democrats responded with criticism of border policies, including Sen. Martin Heinrich of New Mexico.

"Heartbroken and sickened by this news," Heinrich said on Twitter. "I am urgently demanding more details, but the Trump administration must be held accountable for this child’s death and all the lives they have put in danger with their intentional chaos and disregard for human life."

Several critics said the death was especially troubling during the Christmas holiday season.

"The news of another child death in CBP custody is heartbreaking," Sen. Kamala Harris, D-Calif., tweeted. "We must demand a full and thorough account of what happened to this 8-year-old boy."

"Tragic news this Christmas Day," Rep. Nanette Barragán, D-Calif., remarked on Twitter. "Another death of a child in CBP custody? This is unacceptable. It is not ok that children are dying, that they are separated from their parents and don’t receive proper medical screenings and treatment. Congress must act. Real oversight coming 1/3."

Contributing: Lucas Peerman and DylanTaylor-Lehman in Las Cruces, New Mexico, Aaron Martinez/El Paso Times, Kristin Lam and The Associated Press