Many victims of MS-13 came to the United States as unaccompanied minors. Audio: Listen to this story. To hear more feature stories, download the Audm app for your iPhone. Juliana grew up with a single memory of her father. He was sitting in the half-light of evening on the porch of their home, in a small town in El Salvador, while her mother cooked dinner in the kitchen. A man in a black mask emerged from the darkness. Juliana heard three gunshots, and saw her father fall off his chair, vomiting blood. She was three years old at the time, and afterward she wondered if the killing had actually happened. The most tangible detail was the man in the mask, who came to seem more present in her life than her father ever was. Juliana used to find her mother by the windows, pulling back a corner of the curtains to be sure that he had not returned. “It was like that man went on living with us,” Juliana told me. One day when she was older, her mother said that a gang called the Mara Salvatrucha, also known as MS-13, had killed her father for refusing to pay a…