The young men from Guinea had decided it was time to leave their impoverished homeland in West Africa. But instead of seeking a new life in Europe, where so many African migrants have settled, they set out for what has become a far safer bet of late: the United States.

“Getting into the United States is certain compared to European countries, and so I came,” said Sekuba Keita, 30, who was at a migrant center in San Diego on a recent afternoon after an odyssey that took him by plane to Turkey, Colombia, El Salvador and Nicaragua, then by land to the Mexico-U.S. border.