Juan was a policeman in El Salvador, but, ten years ago, when he moved to Brentwood as an undocumented immigrant, he took a job in landscaping. Juan is in his early forties, strapping and chatty, and lives with his wife, Silvia, and their two daughters. He speaks in rapid-fire Spanish except when he mentions MS-13; in several months of conversations, I only ever heard him say the name in English. In March, 2016, Juan’s fifteen-year-old daughter disappeared for three days with a teen-age boy. When Juan called the police, they told him that she had probably just run off with a boyfriend. He and Silvia were circling the neighborhood when they spotted her, in tattered clothes, staggering around at a major intersection. Clearly drugged, she had been dumped on the street.

“What I tell my kids, and also my friends, is that if you meet someone who just arrived here, especially a kid who came alone to Long Island, avoid him,” Juan told me. “There’s a fifty-fifty chance he’s in the gang.” Juan and Silvia began keeping detailed logs of worrisome encounters or observations, such as an unfamiliar car parked on their block or a hostile encounter with aggressive teen-agers at a gas station. They supplemented their records with photographs and videos. When I asked them if they planned to show these materials to the police, they told me it would be useless. “There’s only one group out here that’s actually helping,” Juan said. He was referring to Make the Road New York, an organization that provides support to local immigrants. At least once a week, Juan and Silvia visit the group’s offices to share their concerns and to get legal advice. Walter Barrientos, a thirty-three-year-old from Guatemala who grew up in Amityville, is the lead organizer in Suffolk County. “There are warning signs before there’s gang violence,” he told me. “Girls start disappearing. The police write it off as teen-age romance, but it’s much more serious than that. Not long after someone disappears, even if they eventually return, people turn up dead.”

Last December, Juan and Silvia’s daughter disappeared again. Late one morning, a boy led her from school to a waiting cab. When, a few hours later, she hadn’t returned home, Juan and Silvia went to the school, where a security guard demanded to see identification. “I have a daughter in this school,” Juan protested. He and Silvia presented identification cards from Make the Road, but the guard declared them invalid; eventually, Silvia persuaded him to accept a Salvadoran I.D. card. They met with a school administrator, but she was reluctant to share any information. Juan and Silvia returned home and their daughter was there, unharmed but too scared to say anything about what had happened.

Juan persuaded her to tell him where the boy had taken her: a small, ramshackle house on the outskirts of town. When Juan arrived, a woman answered the door, and insisted that the boy he was looking for didn’t live there. Juan went home and called the police, who promised to investigate. Juan and Silvia never heard from them again. Several months later, Juan, watching the local news, recognized the house he had visited. Two brothers who belonged to MS-13 lived there; they had just been arrested for the murder of Nisa Mickens and Kayla Cuevas. The police found guns, knives, and drugs on the property. “I thank God that my daughter’s O.K., but no one else,” Juan said.

Now Juan’s family travels around Brentwood in his truck, careful not to stray far from one another. The parents and the children are in a precarious situation, but in different ways—the daughters because of the gang and Juan and Silvia because they’re undocumented. Recently, Juan was stopped for a traffic infraction and ticketed for driving without a license. There have been more police officers on the roads since MS-13 became national news, and in recent months Governor Andrew Cuomo has sent state troopers to patrol the streets of Brentwood and Central Islip. For an undocumented immigrant, multiple citations for driving without a license can trigger the involvement of Immigration and Customs Enforcement. In the past year, Juan has spent about a thousand dollars on tickets; he goes to pay them on days when it’s raining, so that he doesn’t miss landscaping work. He told me, “In some ways, the gang members have it easier than we do. If they go to jail, they’re protected by their own. If I went to jail, or got deported, I’d be at risk. I’d be a target. No one would protect me.”