The immigration crisis on the southern border became more personal to New Yorkers last week, when images emerged of small children in masks and backpacks coming and going from a child welfare agency in East Harlem.

It soon became clear that several hundred children separated from their parents out of more than 2,500 in the country had been quietly dropped on New York City’s doorstep. But their identities, the whereabouts of their parents and their future are still unknown to officials in the city.

“There was no game plan for reuniting these kids with their families,” Mayor Bill de Blasio of New York said at a news conference on Monday. He added, “We have no timeline from the federal government, we have no accounting for how many kids are here and where they are.”

Here is what we do know thus far:

Why were the children brought to New York?

New York State already had welfare agencies that had been resettling unaccompanied minors for the last five or so years. Those children — often teenagers — came alone to the United States with the purpose of asking for asylum or finding another way to stay in the country. The latest children have been “rendered” unaccompanied by being separated from their parents, who have been taken into custody for criminal prosecution.