The magnitude of the crisis facing President Biden at the U.S.-Mexico border came into clearer focus Wednesday as the new administration was holding record numbers of unaccompanied migrant teens and children in detention cells for far longer than legally allowed and federal health officials fell further behind in their race to find space for them in shelters.

More than 8,500 migrant teens and children who crossed the border without their parents are being housed in Department of Health and Human Services shelters as they wait to be placed with relatives or vetted sponsors. Nearly 3,500 more are stuck at Border Patrol stations waiting for beds in those shelters to open up, the highest figure ever, according to internal data reviewed by The Washington Post.

Held in grim steel-and-concrete cells built for adults, these young people are spending an average of 107 hours awaiting transfer to an HHS-run shelter, well over the 72-hour legal limit, the data shows. The largest number of unaccompanied minors held this way during the Trump administration was about 2,600 in June 2019, according to congressional testimony and two former Customs and Border Protection officials who were involved in handling that crisis.

The Border Patrol warehouse with chain-link holding pens that were decried as “cages” in 2018 has been closed for renovations, but the conditions in the stations are not much better. Young people are waiting in cramped, austere holding cells with concrete floors and benches. Lights remain on 24 hours a day, agents say, and there are few places to play.

Troy Miller, the acting CBP head, said those housed at the stations have full access to meals, snacks and medical care, as well as showers every 48 hours.

“Many of us, maybe most of us, are parents,” Miller told reporters Wednesday. “I myself have a 6-year-old, and these Border Patrol agents go above and beyond every single day to take care of the children.”

He acknowledged that the Border Patrol continues “to struggle with the number of individuals in our custody, especially given the pandemic.”

Over the first week of March, HHS received more than 450 migrant teens and children per day on average, roughly three times as many as the agency was able to release to family members and sponsors, according to data reviewed by The Post. About 87 percent of the migrant minors in government custody are between the ages of 13 and 17, the latest statistics show.

“As far as HHS, we continue to work with them to move children out of our custody as quickly as we can,” Miller said, “and we need to move them out quicker.”

HHS officials are scrambling to identify new government sites that could shelter migrant teens and children. Officials are scheduled Thursday to visit California’s Moffett Field, a former Navy station in Santa Clara County that now hosts a NASA research center and Google facilities, according to an email sent Wednesday to congressional offices and obtained by The Post. They are also considering Fort Lee, an Army training installation in central Virginia.

Late last month, HHS reopened an emergency facility in Carrizo Springs, Tex., that was used for just one month in 2019 during the Trump administration. It reopened with capacity for 700 teens between the ages of 13 and 17 and can now hold up to 952, according to an HHS notice obtained by The Post. Officials are also looking into reopening a similar facility in South Florida that was called Homestead during the Trump administration but has been renamed Biscayne.

HHS officials have lifted capacity restrictions implemented to lessen the spread of the coronavirus, a move that potentially opens up thousands of additional beds.

Veteran officials at the Department of Homeland Security worry that the influx at the border is building with unprecedented speed, with the potential to be the largest in decades. Homeland Security Secretary Alejandro Mayorkas this week sent a mass email to department employees seeking volunteers to travel to the border and help with what he described as an “overwhelming” number of migrants seeking access to the country.

Biden has quickly reversed or rescinded several of his predecessor’s immigration policies, delivering on campaign promises to make the United States more welcoming and humane while overhauling the nation’s clogged asylum system.

But families and children without their parents are arriving in greater numbers every week, many saying they have heard Biden has eased border controls. A year ago, the Trump administration implemented a public health order that returned nearly all migrants caught at the border back to Mexico. Soon after taking office, Biden stopped turning back children traveling without their parents, and the latest data shows that the administration is no longer sending back most families, especially those with young children.

CBP published enforcement data Wednesday showing that the agency made 100,441 arrests and detentions in February, a 28 percent increase from the previous month. The number of unaccompanied minors taken into custody jumped 61 percent to 9,457, the agency reported, and the Biden administration is on pace to receive a record number of unaccompanied minors this month if trends continue.

“The Biden administration’s border crisis of unaccompanied children being detained at overcrowded Border Patrol stations is a direct result of their undoing the previous administration’s policies with no consideration of the ramifications of removing those policies and how it would incentivize migration,” Sen. Rob Portman of Ohio, the ranking Republican on the Senate Homeland Security and Governmental Affairs Committee, said in a statement. “I hope the administration will change course soon.”

Biden officials have urged migrants not to travel to the border, telling asylum seekers they need more time to rebuild the system. But the message does not appear to be working, with the U.S. ­government competing against smuggling organizations encouraging migrants to leave now, as well as word of mouth from the thousands of migrants who are being processed and released into the United States each week with a notice to appear in court.

“The United States is continuing to strictly enforce our existing immigration laws and border security measures,” Miller said in a statement. “Those who attempt to cross the border without going through ports of entry should understand that they are putting themselves and their families in danger, especially during the pandemic.”

The border is not open, he said: “Do not believe smugglers or others claiming otherwise.”

In private, frustration is building among government agencies that see no end in sight and potentially dangerous overcrowding, especially for teens and children who are not supposed to languish in detention cells. Officials described the surge as “overwhelming,” “on fire” and potentially larger than the 2019 crisis, when CBP took nearly 1 million migrants into custody amid a historic influx of Central American families.

“This surge is going to make the 2019 crisis pale in comparison,” said one official, who was not authorized to speak to reporters and commented on the condition of anonymity.

The 2019 surge ended when Trump hectored Mexico into carrying out a militarized crackdown on Central American migrants and allowing the expansion of the “Remain in Mexico” program requiring asylum seekers to wait outside U.S. territory. More than 70,000 were sent to wait in Mexico under the program, which immigrants’ advocates denounced as subjecting vulnerable groups to dangerous and squalid conditions in border cities.

This month, the Biden administration is on pace to make more than 130,000 detentions and arrests, a volume eclipsed only by the peak of the 2019 surge, when 144,000 were taken into custody. U.S. agents are detaining more than 4,200 people along the border each day, internal data shows.

Biden officials have blamed the rising numbers and their struggle to keep pace on the Trump administration’s deterrent approach to irregular migration, saying they have inherited a broken and inadequate system.

Roberta Jacobson, the veteran former diplomat that Biden has appointed a special adviser on border issues, told reporters Wednesday that the influx is also the result of migration demand that was bottled up during the Trump years, then made worse by the economic impacts of the coronavirus and damaging hurricanes in Central America.

Jacobson said Biden’s team has veteran advisers who have handled previous humanitarian emergencies along the border.

“We’ve seen surges before. Surges tend to respond to hope. And there was a significant hope for a more humane policy after four years of pent-up demand,” said Jacobson, the former U.S. ambassador to Mexico. At times during her briefing she spoke in Spanish to reiterate a message that migrants should not attempt the journey.

“So I don't know whether I would call that a coincidence, but I certainly think that the idea that a more humane policy would be in place may have driven people to make that decision” to migrate, she said. “But perhaps more importantly, it definitely drove smugglers to express disinformation, to spread disinformation about what was now possible.”

The solutions Biden officials have presented are aimed at addressing the long-term “root causes” of irregular migration, rather than the current emergency at the border. Jacobson said the administration is seeking to channel $4 billion into development aid and job creation programs for Central America’s Northern Triangle — El Salvador, Guatemala and Honduras.

Jacobson also said the administration will restore the Central American Minors program, which Biden, as vice president, helped establish during the first major influx of families and children in 2014. That program allows minors with parents living in the United States to apply in their home countries for permission to reunite in the United States, rather than hiring a smuggler and risking the dangerous journey north.

Marianna Sotomayor, Seung Min Kim and Maria Sacchetti contributed to this report.