 The number of unaccounted immigrant children rising amid the surge of migrants and asylum seekers at the U.S. southern border with Mexico. Unaccompanied migrants under 18 can't be deported. Most spend a few weeks in government shelters before they're handed over to relatives or sponsors. Case managers are supposed to follow up. But according to government data obtained by the New York Times, more than a third of the migrant kids released into the U.S. can't be reached.