The first memory Hilda Ramírez has of the United States is the sound of helicopters. Four years ago, she, her eight-year-old son, Ivan, and five other migrants from Central America piled into a small raft on the southern bank of the Rio Grande, the final step in a perilous trip through Mexico that she had begun one week before. When the group crossed the border near McAllen, Texas, they were immediately surrounded by Border Patrol agents, boats and a helicopter circling overhead.