While hundreds of children separated from their parents at the border have been reunited, the U.S. is holding almost 13,000 undocumented migrant children, the New York Times reported Wednesday. According to data obtained by the Times, 12,800 children were in federal custody as of this month, the most ever recorded and more than five times the number of children held last summer. The Times said the huge increase is not because of more migrants, but because federal authorities are not releasing many children to live with families or sponsors. Most of the children arrived at the border unaccompanied by their parents, the Times reported. Detention camps are said to be around 90% capacity, and authorities worry a new influx of migrants could overwhelm the existing system.