Biden administration officials said Friday that they do not have the ability to quickly add thousands of shelter beds to care for the record numbers of migrant teens and children stuck in crowded Border Patrol stations, as the new administration struggles to care for the soaring number of families and unaccompanied minors crossing from Mexico each day.

In recent days there have been more than 3,500 unaccompanied teens and children stranded in steel-and-concrete detention cells designed for adults, waiting for shelter beds to open up, the latest Department of Homeland Security figures show. The minors are spending an average of 108 hours in Border Patrol stations before transfer to shelters run by Health and Human Services, well in excess of the 72-hour limit required by law. More than 130 minors have been waiting 10 days in the custody of U.S. Customs and Border Protection.

Biden officials, who spoke to reporters Friday on the condition of anonymity, said they are working urgently to accelerate the release of the minors to parents and other relatives living in the United States, but this week they have increased their shelter capacity by only about 200 beds.

The refugee office of HHS, which is responsible for taking custody of the minors while eligible sponsors are identified and vetted, is currently receiving three times more teens and children than the number it’s releasing, internal figures show.

The backup has produced a ballooning crisis for the Biden administration at a time when the president is seeking to advance a major immigration bill that would provide a path to U.S. citizenship for millions of immigrants who lack legal status. Biden has promised to quickly rebuild the immigration system and make it more humane, and prominent Republicans have been criticizing his approach. House Minority Leader Kevin McCarthy (R-Calif.) and a group of Republican members plan to travel to South Texas on Monday to assess the crisis themselves.

Biden officials say they are working to address the root causes of irregular migration and rebuild the U.S. asylum system. But they have not proposed new enforcement measures and say they do not have an ability to rapidly expand their shelter network, as most sites require approval from state regulators.

The administration is looking at Fort Lee, a military base in central Virginia, and Moffett Field, a NASA site in California, but HHS must provide Congress with 15 days notification before opening a new temporary facility, officials said Friday. That has not yet occurred.

In a statement Friday, DHS and HHS announced a new memorandum the agencies said would promote “the safe and expeditious transfer and care” of unaccompanied children by more quickly placing them with relatives or parents already living in the United States.

Under the Trump administration, HHS signed an agreement to share information about potential sponsors with ICE, which the agencies said had a chilling effect on the willingness of undocumented family members to claim unaccompanied children for fear of being deported. Biden officials said they halted the practice. Roughly 87 percent of unaccompanied minors are released to a parent or another close relative.

The administration is also scrambling to find staff who can help CBP handle the surge. On Thursday night, U.S. Immigration and Customs Enforcement sent an urgent email to senior staff asking for volunteers to quickly deploy to U.S. border stations and tent sites where holding cells are crammed beyond capacity.

“This situation mandates immediate action to protect the life and safety of federal personnel and the aliens in custody,” wrote Michael W. Meade, ICE’s acting assistant director for field operations, in an email obtained by The Washington Post.

“Start and end dates are TBD, but could begin as soon as this weekend at locations along the SWB, most likely Texas,” Meade wrote, referring to the southwest border. He also told staff the emergency conditions could persist, adding, “It is anticipated that the enforcement actions will continue to grow over the coming months.”

The number of migrants crossing the U.S.-Mexico border shot up to more than 4,200 per day this week, nearly double the volume in January, the most recent statistics show. CBP is on pace to make more than 120,000 arrests and detentions in March, which would make it one of the busiest months in recent decades.

Meade told senior ICE officials that CBP needs immediate help guarding families and minors who crossed the border without their parents.

DHS Secretary Alejandro Mayorkas sent out a call earlier this week to other DHS agencies for volunteers to assist CBP with what he called “an overwhelming number of migrants seeking access to this country along the Southwest Border.”

Mayorkas noted that more than 900 volunteers from across DHS traveled to the border during the 2019 crisis, when CBP detained nearly 1 million migrants amid a historic influx of families. His request was aimed primarily at employees capable of performing administrative support functions, not law-enforcement-related duties.

One official who received the email said it was the first time ICE’s Enforcement and Removal Operations branch has received a call for volunteers to deploy to the border. ICE deployments during previous migration waves were mandatory, and officers typically aid CBP by helping facilitate the release of migrants with GPS monitoring devices and other logistical needs.

ICE officials referred inquires to DHS, which did not immediately respond to a request for comment.