The US Border Patrol has detained more than 11,000 unaccompanied migrant children between February 28 to March 20, according to preliminary government data reviewed by CNN, already eclipsing the number of minors apprehended in the full month of February. In recent weeks, the number of encounters of children crossing the US-Mexico border alone has continued to climb, overwhelming the government’s resources that had already been under strain because of the coronavirus pandemic. In all of February, Border Patrol apprehended nearly 9,300 unaccompanied children at the US southern border, up from around 5,600 in January. The number of detentions will also likely surpass, according to the preliminary data, the high during May 2019 when around 11,400 unaccompanied children were arrested on the US-Mexico border. The latest figures indicate the challenge the Biden administration is up against as migrants continue to approach the US-Mexico border, even though officials have warned them not to come and despite the continued use of a Trump-era policy put in place during the pandemic that allows border officials to turn away single adults and some families. While the growing number of children is a pressing challenge for the administration, kids crossing the border alone still make up the minority of encounters, the preliminary data reveals. Between February 28 and March 20, Border Patrol apprehended more than 60,000 single adults and more than 32,000 families, according to the data. The Biden administration has pushed back against characterizing the flow of migrants as a “crisis,” maintaining instead that it’s a challenge. Over the weekend, Homeland Security Secretary Alejandro Mayorkas defended the administration’s work at the southern border and placed blame on the Trump administration for dismantling the immigration system, saying his department now has to rebuild it “from scratch.” President Joe Biden told reporters outside the White House Sunday he would visit the border “at some point” and maintained, “I know what’s going on in those facilities” when asked if he wanted to see the situation first hand. Photos released by Democratic Rep. Henry Cuellar of Texas on Monday provided a rare glimpse of the crowded conditions in border facilities. Images revealed dozens of children and adults in an overflow facility in Donna, Texas, which was set up earlier this year to help process migrants. White House press secretary Jen Psaki said Biden does not think the conditions at Customs and Border Protection facilities are acceptable, and that the administration is working on moving children out of those facilities as soon as possible. “They are not meant for children and that’s why he wants to open more shelters and increase and expedite processing at the border, and that’s an issue he’s focused on every single day,” Psaki told CNN’s Jake Tapper during an interview on “The Lead” on Monday. Administration officials, meanwhile, have been racing to find suitable shelter space for unaccompanied minors, resorting to convention centers and emergency intake sites to transfer them out of Border Patrol facilities, which are intended to process adults, not care for children. The San Diego Convention Center, best known for hosting the annual Comic-Con International convention, will soon be used to house unaccompanied migrant children, the city and county announced Monday. The Health and Human Services Department is already using part of the Kay Bailey Hutchison Convention Center in Dallas to shelter about 1,500 children, though the space has capacity for 2,300 children. HHS also announced the opening of a new influx facility in Texas to accommodate approximately 500 children, with the potential to expand to 2,000. There are about 10,500 children in the department’s custody. As of Monday, 81 members of the Federal Emergency Management Agency were supporting HHS in sheltering the unaccompanied children, according to the agency. FEMA personnel are present in Dallas, Midland, Texas; Donna,Texas; San Diego and Washington, DC. Federal law requires migrant children to be turned over to HHS, which is charged with their care, within 72 hours. But amid constraints resulting from the pandemic and the accelerated pace of arrivals, the administration has had a difficult time catching up to the arrival of minors, resulting in thousands held up in border facilities. The average time in custody for unaccompanied children continues to hover around 130 hours, exceeding the 72-hour legal limit, though the number of children in Customs and Border Protection custody dropped slightly to just under 4,900, documents reviewed by CNN show. To fast track the process for minors so that they can be reunited with a relative in the US, the Office of Refugee Resettlement, a federal agency under HHS, released field guidance allowing the expedited release of some unaccompanied children in custody, according to a memo obtained by CNN. Children who meet certain requirements will be released to their parents or legal guardians under this revised policy, including if the child is determined to not be especially vulnerable, if there are no “red flags” present in the case, like abuse or neglect. Earlier this month, the administration also terminated a Trump-era agreement that it said discouraged sponsors of unaccompanied migrant children, like parents or relatives, to come forward due to fear their information would be shared with immigration enforcement agencies. This story has been updated with additional background information.