But in recent days, hundreds of migrants from another part of the world have caused city officials already busy with one immigrant surge to scramble on a new and unexpected one. Men, women and children from central Africa — mostly from the Democratic Republic of Congo and Angola — are showing up at the United States’ southwest border after embarking on a dangerous, monthslong journey.

Their arrival at the border and at two cities more than 2,100 miles apart — San Antonio and Portland, Me. — has surprised and puzzled immigration authorities and overwhelmed local officials and nonprofit groups. The surge has prompted Portland to turn its basketball arena into an emergency shelter and depleted assistance funds meant for other groups. Officials in both cities have had to reassure the public that fears of an Ebola outbreak were unfounded while also pleading for volunteer interpreters who speak French and Portuguese…

But many of the recent African migrants do not have relatives in the country, so they are being released with no travel arrangements, a problem that local officials and nonprofit groups are forced to sort out. Some of the Congolese migrants in San Antonio said Border Patrol agents had chosen their destination cities for them, or encouraged them to select one of two cities, New York and Portland.