“We can’t exclude a kid because we suspect they are in a gang,” Mr. Loeschner said. “That’s state and federal law that they are entitled to an education.”

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Even before the girls’ murders, students were subject to random screenings with metal detectors, which have increased over the past few weeks, he added. There are no detectors at the entrances of either high school, however.

Some parents were concerned that the school’s response to the violence was not proactive enough. Levi McIntyre, the school superintendent, sent an email to parents warning their children not to wear royal blue, the color identified with MS-13, or clothes displaying the Salvadoran flag. A student on the way to school, he wrote, recently had his blue shirt torn off by gang members and burned.

MS-13 was formed in Los Angeles in the 1980s by immigrants from El Salvador escaping civil war. The abbreviation stands for Mara Salvatrucha, which roughly translates to “Salvadoran street posse.”

The authorities say the gang has been in Suffolk County since around 1998, and is organized in cliques bearing names like the Brentwood Locos Salvatruchas. Leaders gather to discuss their lines of business — extortion, prostitution, robbery, drug dealing — and to authorize the killings of chavalas, or members of rival gangs like the Bloods and the Crips, court papers say.

In 2009, a 15-year-old boy, Christopher Hamilton, was fatally shot in the head after an MS-13 crew in search of chavalas opened fire with rifles and handguns on a house party on American Boulevard here.

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Two years later, an 18-year-old Brentwood man was fatally shot in his driveway, and a 22-year-old local leader of MS-13 was convicted of the killing.

“In the past, it used to be like rival gangs on each other,” Dr. McIntyre said. “But now it has taken another turn. When it goes after all kids, it’s a whole new realm. It’s tearing the fabric of our community apart.”

Noel Vega’s son was a classmate of the murdered girls, who wondered whether he could be next.

“He’s more upset about the fact that they keep finding bodies,” Mr. Vega said, standing outside a Brentwood funeral home for Kayla’s wake with fellow members of the Christian Motorcyclists Association. They are not the only group offering unofficial security to the town; he noted that the crisis even brought the Guardian Angels to Brentwood.

Of his son, Mr. Vega added: “He actually wants to move out of Brentwood; he wants to move out of state. He’s upset and he fears for the loss of his friends and himself. It gets me upset; we all get upset.”

The recent murders have exacerbated disputes in the town over immigration policy, which Donald J. Trump, the Republican candidate for president, fueled during last week’s debate by saying that the gangs roaming the streets were made up of illegal immigrants.

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“There’s been a huge influx, to be honest with you,” said Ray Mayo, the president of the Brentwood Association of Concerned Citizens, who added that he was upset over undocumented immigrants crowding rental properties. “It seems like a whole new set of gang members who have stirred the pot up.”

Two law enforcement authorities, who spoke on the condition of anonymity because of the continuing murder investigation, said that over the last several years the gang has sought to enlist recent immigrants from Central America because they are often more vulnerable to recruitment.

But some recently settled families are just as worried about their own children’s safety.

“I am afraid, as a Salvadoran,” said Ana, 38, a mother of two girls, one in high school. She fled El Salvador in 2006 and has since become a member of Make the Road New York, an immigrant activist group. She did not want to give her full name for fear of retribution.

“It makes me feel bad that people think this of all Salvadorans,” she said. “Violence was the reason I left — when they killed my brother. And now we are experiencing the same violence.”

Distrust of the Suffolk County police among Latinos is palpable and long documented. Residents said they were dismayed by a dearth of Spanish-speaking officers, and undocumented immigrants in particular often worry that if they report information, the authorities will turn them over to immigration officials.

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Mr. Sini said that would not happen, and that he was trying to reassure immigrant communities to work with the police.

Ms. Rodriguez, whose parents came from Puerto Rico, said that two years ago, when gang members threatened Kayla on a friend’s block, she went to the police.

“I got attitude like they were talking to somebody off the street,” Ms. Rodriguez said. “They wouldn’t even report it,” she added. “They told me to tell her: ‘Don’t go on the block.’”

The feeling of helplessness is spreading among the teenagers.

At a vigil held for the murdered girls before a football game, some students held signs: “Help Us!” “Stop the Violence!” Others shook their heads when Mr. Sini told students to call a hotline for investigative tips.

“We’re the ones out here, dealing with it all,” said a 16-year-old boy who would give only his nickname, Tiny T. “They think they can do something, but they’re just fooling. They can’t do nothing.”

At Kayla’s wake, a 17-year-old student too afraid of MS-13 to give his name said: “You don’t know who’s watching you, who’s following you. Just yesterday, a group of guys in a car with blue bandannas followed a girl home in Brentwood.”

He, his mother and his cousin wore T-shirts that read “Justice for Kayla,” which they had printed at the mall. “Afraid?” his cousin, a 19-year-old woman, said. “There’s not even a limit to afraid.”

At memorials for both Kayla and Nisa, on the cul-de-sac near where their bodies were found, basketballs sat among the glass candles and deflated balloons. Kayla, a tenacious athlete, was going to try out for the varsity basketball team this year. Instead, her mother was starting a scholarship fund called Ball Is My Life.

Ms. Rodriguez hoped her daughter’s death would at least stop the cycle. “It can’t go on anymore,” she said.