On Monday, a 16-year-old migrant boy became the fifth migrant child to die after crossing the US-Mexico border since December.

Experts and advocates have decried the Border Patrol stations known as "hieleras," which migrants have alleged are freezing cold, with inedible food, undrinkable water, and open toilets, are unsuitable for children.

More recently, migrants have been held in even shoddier conditions in the stations' parking lots, where they sleep in tents and on the ground, in open-air enclosures.

But the Trump administration has said the surge in the number of families with young children that have been crossing the border recently are the main driver of the deaths, acknowledging that the government facilities weren't built to shelter sick children.

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The deaths of five migrant children in recent months have triggered an uproar over the US government's practice of detaining young children after they cross the border, and caused advocates to question the medical care kids are receiving in custody.

But experts say the problem began long before President Donald Trump — and that the recent deaths in Border Patrol custody have been years in the making.

On Monday, 16-year-old Carlos Gregorio Hernandez Vasquez died in a Border Patrol station in Texas' Rio Grande Valley, six days after he crossed the US-Mexico border, according to the Customs and Border Protection agency. He was diagnosed with Influenza A while in custody, officials told The New York Times.

Though Border Patrol's own rules stipulate that migrant children be released from the facilities and transferred to shelters within 72 hours, Carlos was detained for twice that length of time.

Officials told The Times that Carlos did not show any signs of illness during a health screening when he was first detained, but that a nurse practitioner diagnosed him with influenza on Sunday and recommended the antiviral drug Tamiflu, which Border Patrol agents picked up from a nearby pharmacy.

"The men and women of US Customs and Border Protection are saddened by the tragic loss of this young man and our condolences are with his family," the acting CBP commissioner, John Sanders, saidn in a statement. "CBP is committed to the health, safety and humane treatment of those in our custody."

This Dec, 12, 2018 photo Felipe Gomez Alonzo, 8, near his home in Yalambojoch, Guatemala. He died in US custody. Catarina Gomez via Associated Press

Vasquez is the latest of several migrant children's deaths that have occurred since December. Jakelin Caal Maquin, 7, died on December 8, roughly 24 hours after Border Patrol detained her. Felipe Gomez Alonzo, 8, died late on December 24, after spending nearly a full week in various Border Patrol facilities.

Two other children recently died after crossing the border, though neither were in Border Patrol custody at their time of death. A two-year-old migrant boy died in the hospital on May 14, weeks after he and his mother were released from custody. Another 16-year-old boy died April 30 in a migrant children's shelter operated by the Department of Health and Human Services.

Trump has previously blamed Democrats for the deaths.

"Any deaths of children or others at the Border are strictly the fault of the Democrats and their pathetic immigration policies that allow people to make the long trek thinking they can enter our country illegally," the president tweeted in December. "They can't. If we had a Wall, they wouldn't even try!"

Experts say the facilities are unsuitable for children

Migrants are seen outside the US Border Patrol McAllen Station in a makeshift encampment in McAllen, Texas on May 15, 2019. Reuters/Loren Elliott

Jakelin and Felipe were eventually brought to hospitals for care, but the temporary facilities that they were initially detained in have come under particular scrutiny for what experts have described as poor conditions that are unsuitable for children.

"They're designed to be problematic and not safe," Anne Chandler, the executive director of Tahirih Justice Center's Houston office who's been doing this work for 20 years, told INSIDER in December, after the first two deaths.

She added: "They are designed and meant for punishment and deterrence, and this is nothing unique about this administration. These [Customs and Border Protection] facilities have been nasty and ugly since I started this work."

Since December, Border Patrol has come under even more scrutiny over the chaotic and haphazard conditions of its facilities, as authorities struggle to process the influx of families that has overwhelmed the stations. Border Patrol has even begun detaining migrants in the stations' parking lots, where migrants sleep on the ground, with only tents and thin mylar blankets to shelter them.

Read more: Aerial photos show migrants sleeping on the ground in makeshift tent encampment at a Texas Border Patrol station

Migrants are seen outside the US Border Patrol McAllen Station in a makeshift encampment in McAllen, Texas on May 15, 2019. Reuters/Loren Elliott

Migrants have long complained about the conditions in the Border Patrol facilities where they're taken immediately after their arrests. They have nicknamed the stations "hieleras," Spanish for "icebox," because of the freezing temperatures.

Dozens of migrant children and their parents submitted sworn declarations alleging they had experiences involving inedible food, undrinkable water, overcrowding, and few opportunities to shower or clean themselves.

Those declarations were filed over the summer as part of a long-running lawsuit over the US government's treatment of detained migrant children. CBP defended itself by pointing to a government report filed last June that declared the agency "continues to comply" with court-ordered rules governing how to treat migrant children.

But Colleen Kraft, president of the American Academy of Pediatrics, told INSIDER in December that she wasn't surprised when she learned of the deaths of Jakelin and Felipe after they were detained.

"We've seen this coming," she said. "When you take a child and you put them into a facility that's cold, and they don't get proper sleep because the lights are on, and they have risk of infection from open toilets, it's a really bad combination of things that could really result in some very sick children."

'I've never seen a criminal baby'

A picture of Jakelin, a 7-year-old girl who died in US custody. Reuters/Jose Luis Gonzalez

One unique problem the Trump administration is facing is the sheer number of families with young children crossing the border to seek asylum.

In recent months, border apprehensions have skyrocketed to levels not seen in 12 years, and the type of migrants coming to the US has evolved over time — posing its own set of challenges.

In previous decades, most of the immigrants crossing the US-Mexico border were Mexican men entering alone, seeking work. But now, more and more Central American families and unaccompanied children are crossing together in large groups, often directed by smugglers.

The Trump administration has pinned much of the blame on the migrants themselves, urging parents to avoid taking their children on long, dangerous journeys to the US, where they will then spend at least several days in detention.

"The unprecedented number of families and unaccompanied children at the border must not be ignored," former Homeland Security Secretary Kirstjen Nielsen said in a statement in the wake of Jakelin and Felipe's deaths. "I once again ask — beg — parents to not place their children at risk by taking a dangerous journey north. Vulnerable populations — including family units and unaccompanied alien children — should seek asylum at the first possible opportunity, including Mexico."

Read more: After a 7-year-old migrant girl died in Border Patrol custody, Kirstjen Nielsen said 'this family chose to cross illegally', and critics are outraged she's blaming the death on the family

Acting Secretary of Homeland Security Kevin K. McAleenan speaks about the impact of the dramatic increase in illegal crossings that continue to occur along the Southwest during a news conference, in El Paso. Reuters/Jose Luis Gonzalez

But Kraft said the migrants are "fleeing violence, and death, and recruitment into gangs," and don't make long, difficult journeys to the US on a whim.

"I've seen a need to try to tone down the rhetoric on people coming to the border as 'criminals,'" Kraft said. "Half of them are children. And I've never seen a criminal baby."

Kraft said she was heartened when then-CBP Commissioner Kevin McAleenan reached out to her in December asking for assistance. CBP confirmed to INSIDER that the discussion took place, and said McAleenan would continue to seek AAP's input going forward.

"The fact that he reached out to us is very positive," she said. "We have 67,000 medical experts — use our expertise. Allow us unfettered access to these facilities. Allow us to train your personnel. Allow us to monitor and make recommendations on these conditions, and we can help you out."

After Felipe died, DHS asked the US Coast Guard, the Department of Defense, and the Centers for Disease Control and Prevention to help provide healthcare to migrant children in its custody, partly because the agency didn't have enough trained personnel to handle all the cases itself.

'Why do we have these kids sleeping on cement?'

Detained migrants in US government custody in McAllen, Texas on June 17, 2018. Customs and Border Protection's Rio Grande Valley Sector via Associated Press

Trump administration officials have acknowledged that the holding facilities were originally designed for adult men — not families with young, sick children.

But despite Border Patrol's own rules stating that migrants should generally not stay in holding facilities longer than 72 hours, Chandler said she routinely encounters migrant children like Felipe who have been held much longer.

Chandler said the conditions in Border Patrol stations may not always cause a child to fall sick — they are often already in "bad shape" after dayslong or weekslong journeys to the US through the desert with little access to food or water.

But she added that the Trump administration could implement simple measures to avoid worsening their conditions.

"Why do we have these kids sleeping on cement? We can't afford, as a nation, some type of warm beverage and warm food? These are children, right? It's cruel and unhealthy to say the least," she said. "If kids are coming over and they're sick, that is only going to get worse through this structure."

Migrants are seen outside the US Border Patrol McAllen Station in a makeshift encampment in McAllen, Texas on May 15, 2019. Reuters/Loren Elliott

But part of the blame, she added, lies not just with the facilities, but on broader border policies the US government has implemented for decades. Increased fencing, heightened surveillance technologies, and expanded Border Patrol staffing have pushed migrants away from crossing in heavily patrolled areas.

Jakelin, for instance, had crossed with a group of 164 migrants in a remote part of the New Mexico desert, where Border Patrol staff struggled to accommodate them.

According to a government timeline, the "remoteness" of the area where Caal was detained meant that an hours-long bus trip to a different Border Patrol facility was "the best means to provide the child with emergency care."

"We put in measures to try to hamper the abilities of individuals to cross our border, pushing individuals into more desolate areas," Chandler said. "Most of the time these immigrants present themselves to CBP border people, but when they are pushed into these more remote areas to cross the border, their vulnerability and their health situation escalates."