WASHINGTON (AP) - The U.S. government picked up nearly 19,000 children traveling alone across the Mexican border in March, authorities said Thursday, the largest monthly number ever recorded and a major test for President Joe Biden as he reverses many of his predecessor’s hard-line immigration tactics.

A complex mix of factors in the United States and Central America drove the increase. It has coincided with the Biden administration’s decision to exempt unaccompanied children from pandemic-related powers to immediately expel most people from the country without giving them an opportunity to seek asylum. Children are instead released to “sponsors” in the U.S., usually parents or close relatives, while being allowed to pursue their cases in heavily backlogged immigration courts.

The Border Patrol encountered 18,663 unaccompanied children in March, well above previous highs of 11,475 in May 2019 and 10,620 in June 2014. The agency started publishing the numbers in 2009. Before then, adults made up the vast majority of those crossing the border.

March’s count was roughly double the number of unaccompanied children encountered by the Border Patrol in February and more than five times the number in March 2020.

The huge increase in children traveling alone - some as young as 3 - and families has severely strained border holding facilities, which aren’t allowed to hold people for more than three days but often do. It’s left the government scrambling to find space and hire staff to care for children longer term until they can be placed with sponsors.

For many, a hurricane that hit Central America in November added urgency to endemic poverty and violence that have led people to flee for decades. Changes in U.S. policy under Biden also have guided their decisions, whether real or rumored.

Amid the growing numbers, more than 4,000 people at a U.S. Customs and Border Protection holding facility have been jammed into a space designed for 250 at a tent complex in Donna, Texas. They lay inches apart on mats on the floor with foil blankets.

CBP must transfer unaccompanied children within 72 hours to the U.S. Department of Health and Human Services, whose facilities are more suited to longer-term care while arrangements are made to release them. More than 2,000 children were held longer than that at the Donna facility one day last week, with 39 there at least 15 days.