It's a fast-moving situation, described by the Biden administration as a "stressful challenge" and decried by critics of immigration as a "crisis." There are a lot of developments we're still learning in real time. Here's a look at some of the key numbers we've been hearing, the deeper stories behind them and the questions we have. 3,400 What this number represents: How many unaccompanied migrant children were in Customs and Border Protection custody as of Tuesday The deeper story behind it: This is a higher number than we've seen before. At the peak of the 2019 border crisis -- when there were overcrowded facilities and children sleeping on the ground -- there were around 2,600 unaccompanied children in Border Patrol custody, a former CBP official told CNN. The treatment of kids in custody is one of the thorniest issues at the border. One of the largest public outcries we heard during the Trump administration came when monitors revealed squalid conditions inside CBP facilities where children were held. We haven't heard much about the current conditions in these facilities. But it's concerning, because the number of children arriving is outpacing the Biden administration's ability to place them in shelters overseen by the Department of Health and Human Services. And due to limited capacity at shelters, children are being held in CBP facilities beyond the 72-hour limit the law requires. The questions we have: Will the Biden administration open new facilities to house these children? Is this record high number of kids in CBP custody a sign of a major shift? Or will we see numbers stabilize as the new administration finds its footing?