 The number of unaccompanied immigrant children continues to rise amid the surge of migrants and asylum seekers who cross the border from Mexico into the United States. 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.