The Border Patrol checkpoint outside Alamogordo, N.M., sits on the edge of a forbidding desert of white gypsum sands so remote that the Defense Department routinely conducts missile testing in the area.

Like most Border Patrol stations, the checkpoint is not set up to house children. But 8-year-old Felipe Alonzo Gomez and his father were there on Christmas Eve, and the boy was growing sicker.

The pair had been caught entering the United States illegally Dec. 18, a few miles west of a legal border crossing in El Paso, Texas. Border agents say they were given hot food, juice, snacks and water. For the most part, Felipe seemed fine as agents moved him and his father from one station to another, including late at night, as holding cells filled up.

But then the boy started to cough, his eyes glassy. He was taken to the hospital, where his fever spiked to 103 degrees. After giving him doses of antibiotics and ibuprofen, the hospital released him to Customs and Border Protection.

By the time Felipe and his father were brought to the highway checkpoint near the White Sands National Monument, they had been in custody for six days — double the 72 hours Border Patrol standards recommend.

Over the next several hours, the boy vomited and appeared to be sluggish. He lost consciousness on the way back to the Gerald Champion Regional Medical Center in Alamogordo, where he became the second migrant child to die this month in federal custody. On Dec. 8, Jakelin Caal, 7, died of dehydration and shock at an El Paso hospital about 27 hours after she and her father were taken into custody at a remote border crossing.

The deaths have shaken Border Patrol agents, migrants and advocates and have ignited fresh concerns that a political impasse over border security is endangering children. Federal officials on Wednesday called the deaths a tragedy, noting that until this month no child had died in border custody for more than a decade; they said the Department of Homeland Security is investigating.

Six adults died in CBP custody in 2018, officials said.

House Democrats, who are poised to take control of the congressional chamber next month, said they plan to examine the deaths and the conditions under which young migrants are being held as President Donald Trump attempts to stanch the flow of asylum-seeking families across the border. In a statement, House Minority Whip Steny Hoyer, D-Md., called on Congress to "ask serious questions about what happened and who bears responsibility."

House Minority Leader Nancy Pelosi, D-Calif., called the death "unconscionable," and Rep. Joaquin Castro, D-Texas, the incoming chairman of the Congressional Hispanic Caucus, said it was indicative of a "systemic crisis" within the CBP.

Homeland Security Secretary Kirstjen Nielsen said Wednesday that she had deployed teams from the U.S. Coast Guard's medical corps to the southern border to ensure that migrant children held by the government receive double health screenings. She said she has also asked the Centers for Disease Control and Prevention to investigate the source of what officials called an increase in sick migrants taken into custody. Dozens have been taken to hospitals with flulike and other symptoms in recent days, officials said, raising the question of whether illness may be spreading in migrant shelters in Mexico.

Nielsen will travel to the border this week to see the screenings and conditions at Border Patrol stations. She urged Congress to provide additional funding to handle an influx of immigrant families, who are overwhelming temporary holding cells that were designed to contain single men. And she called on would-be migrants to not attempt to cross the border, especially with children.

"I once again ask — beg — parents to not place their children at risk by taking a dangerous journey north," she said in a statement that urged migrants to seek refuge in Mexico.