Nisa Mickens, 15. Beaten to death. Her body found Sept. 13 by a passing motorist.

Kayla Cuevas, 16. Beaten to death. Her body left in a wooded area not far from homes and found Sept. 14.

Oscar Acosta, 19. Cause of death not released. His remains left in the woods by an industrial area and discovered Sept. 16.

Miguel García Morán, 15. Cause of death not released. Skeletal remains discovered Sept. 21 in the woods off the same industrial area.

All four were Brentwood High School students. Their names are and ever will be memorialized in sorrow — no diplomas, no higher education, no opportunities or careers, no families of their own. Police have said they were killed by street gangs bent on leaving their mark in blood.

The violence has torn the fabric of Brentwood, a hamlet of about 60,000 residents spread over nearly 11 square miles and home to Long Island’s largest school district. Its enrollment of roughly 20,000 rivals the size of a small city. More than 80 percent of the student body are Latinos and in excess of 85 percent are classified as economically disadvantaged, according to state Education Department data.

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So great is the potential for gang assaults that schools Superintendent Levi McIntyre warned parents against letting their children “wear clothing that could be considered to be gang-affiliated.”

The caution, via a robocall and a letter posted on the district’s website, came Sept. 21, on the day a ninth-grader headed to the Freshman Center was approached by “a few individuals in a red car” who took his light blue T-shirt, set it on fire and warned him not to wear that color.

The next day, one parent posted a despairing comment on Facebook’s “Brentwood Moms & Dads Group.”

“I can’t handle much more,” she wrote. “Just received a message from my son at the HS. It says ‘Apparently if you are seen wearing blue today people are going to go after you.’ He’s not but I just want to run and pick him up anyway. This is ridiculous that kids don’t feel safe, parents don’t feel safe. We just want them to get an education . . . now we have to pray they are returning home safe at the end of the day. I feel like I’m in a war zone.”

Among the single-family homes with green lawns and main roads peppered with strip malls and wooded areas, people say they are scared for their lives and those of their children as never before. Several gangs operate in the region — MS-13, the Bloods, the Crips and the Latin Kings — and there are cliques within the gangs as well, law enforcement officials have said.

“I was talking to a gentleman who lives just a few blocks from where the girls were found,” McIntyre said to residents who packed a community forum in neighboring Central Islip on Sept. 28. “I said to him, ‘What’s happening in the neighborhood,’ and he said every single person in the neighborhood is afraid . . . How could they be so close and somebody’s being beaten and no one knows?”

Carolina Frías, 38, whose older daughter graduated among the top students in Brentwood, said she worries constantly about her younger child. She thinks about the danger her 15-year-old daughter may be in and feels relief when she returns home safely.

“The fact that the gangs are there is not a secret for anyone,” said Frías, a school bus driver. “I am praying to God every day . . . and I ask the girl every day how was her day at school because I live with the paranoia that someone is going to bother her.”

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She and her spouse, Francisco Hernández, 46, a machine operator at a factory, said they are planning to move to Florida because they don’t feel safe in Brentwood.

“We have always had crime,” Hernández said in Spanish, “but we haven’t seen the likes of these attacks before.”

The immense high school enrolls 4,700 in grades 10 through 12, split between the Ross Center and the Sonderling Center. In the Freshman Center, about a mile-and-a-half away, are 1,100 ninth-graders. The system has four middle schools, their names like those on a compass — East, West, North and South — as well as 10 elementary schools and a kindergarten center.

Mickens and Cuevas, best friends, both were juniors at the Ross Center. Their bodies were found about a mile-and-a-half from each other. Suffolk police said they were victims of a “brutal attack” and suffered blunt-force trauma.

With the investigation of the two teenagers’ slayings underway, police searched a wooded industrial area in Brentwood and discovered the remains of two other teenagers who had been missing for months. The deaths of Acosta and García Morán also were classified as homicides, though police have not disclosed the manner of their deaths.

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Acosta was a Ross Center student and García Morán was enrolled at the Sonderling Center.

School officials have increased their vigilance for suspicious activity, instituting random checks of classes through metal detectors, approaching students who are seen wearing bandanas, especially in colors associated with gangs, or who flash hand signs that identify their affiliation. Those students are told to change their clothing or correct their behavior, or are sent home.

“It’s been a very difficult few weeks,” said Richard Loeschner, principal of the Ross Center. “The students are pretty shaken about all of the incidents and obviously many of them are still frightened, and the staff as well.”

Loeschner said the school has identified students who were friends of the victims, and counselors are offering them additional support. The building has more security and police officers from the Third Precinct are watching the school’s surroundings.

He is working to identify motivational speakers who can help the students focus on their future rather than their fears.

“We are somewhat getting back to normalcy, but I think this shadow will hang over us probably for many months, if not the school year,” the principal said.

Keeping their children safe from gang violence has been an ever-present worry over the years, said Deborah Alvarado, 46, the aunt and godmother of Nisa Mickens.

She doesn’t have children of her own but has helped to rear her nieces and nephews — “I raised all the babies,” she said. She and her sister — Nisa’s mother, Elizabeth Alvarado, 48 — always had a plan in place to keep track of the children. They would call often and drive the kids everywhere.

Deborah Alvarado, who lives in Bay Shore, said she first realized how such violence could affect her family several years ago, when she took her nephew shopping for school clothes and learned that several colors could identify the teenagers wearing them as belonging to specific gangs — a dangerous tag if they ran into an opposing group.

“He’s telling me, ‘I can’t wear certain colors.’ And I stopped and said ‘Really, Joe?’ and he said ‘Yeah,’ ” she said. “It’s reality.”

Alvarado said she is vigilant, including about her own safety. She talks to friends and family on her cellphone until she gets to her front door and she scouts out cars that drive by on the street — just to make sure she recognizes the people inside.

Things have become more confusing since her niece was killed, she said. It’s hard to know whose support to accept and what could have gone differently.

Her sister Elizabeth, she said, “goes to the store, people stop to hug her. She’s not afraid to hug anybody. She feels the love. I try to keep my sister near. We don’t know who did this. She’s so loving with everybody, but we just don’t know, so I want to protect her.”

Manny Troche, a Brentwood firefighter for 12 years, has seen much destruction in his line of work.

He said he put up a brave face for his 16-year-old daughter, but after discussing the killings with her one night he couldn’t hold in the fear and sadness. After she went to bed, he wept.

“Those four kids, I felt like they’re mine,” Troche, 53, said at the community forum attended by dozens of residents as well as community leaders, school and police officials.

“At the end of the day, all I can bring up is four kids that have died and what are we doing about it?” he said.

The killings bear the trademark of violent street gangs, law enforcement sources have said.

Suffolk County Police Commissioner Timothy Sini, who confirmed last week that the four slayings are believed to be connected to street gangs, said the department is working feverishly to solve the slayings and end the gang scourge. He would not identify any group by name.

Another law enforcement source has said the street gang known as MS-13 could be linked to the recent spate of violence.

That group, also known as Mara Salvatrucha, Central American slang for “Salvadoran gang,” started in Los Angeles but has taken root in El Salvador and in immigrant communities throughout the United States, according to law enforcement sources. Brentwood and surrounding neighborhoods are home to a large enclave of immigrants from Central America, including children from a recent influx of unaccompanied minors who in many cases came to Long Island after fleeing the ruthlessness of those gangs.

Raymond Olivencia, a gang investigator in the office of Suffolk County Sheriff Vincent DeMarco, said at least five cliques of MS-13 groups — including one named after Brentwood — have been identified in the county jail.

Rival gangs of the Bloods, the Crips and the Latin Kings also operate in the region, the sheriff said, and can be brutal in their turf wars. A Crips group also is named after a Brentwood park.

“It’s about making a name for themselves,” Olivencia said. “They all want to become the top guys. They want a lot of respect from everybody . . . and there’s drugs, weapons. They do everything.”

Transnational groups such as MS-13 have even been known to take orders to cause mayhem from their leaders abroad, he said.

Sini, who met with residents at the recent community meeting, said the department has increased its uniformed police presence in Brentwood while also sending officers door-to-door to more than 140 homes near problem areas.

Much of what the department is doing may not be immediately visible yet, he told Newsday.

The department’s criminal intelligence section has compiled “strategic subject lists” that identify active gang members, Sini said. Gang-enforcement officers at the precinct level and members of the department’s firearms-suppression team have been assigned to monitor those subjects.

The department, he said, has “made several arrests of these known gang members” and is working with the FBI and the U.S. attorney’s office to prosecute as many as possible under a federal anti-racketeering statute known as RICO, which allows for a wider range of charges and stiffer criminal penalties.

Police investigators are identifying young people who are not gang members but are thought to associate with them to send them a warning letter “laying out the criminal consequences of engaging in criminal behavior,” Sini said. The department will put in place a gang-prevention program starting at the sixth grade, in partnership with the schools and the nonprofit STRONG Youth.

These are “long-term strategies, and the message that the Suffolk County Police Department and I want to make clear to the people of Brentwood is that we are in this for the long haul,” Sini said. “It may take some time but we are going to solve these cases and our goal is to eradicate gangs in Brentwood.”

DeMarco, who said “gangs are the biggest threat in public safety in Suffolk County,” would like to see the establishment of a multiagency task force involving the police, his office, state troopers and the district attorney’s office, working with federal officials.

“It’s pretty obvious now that the gangs are getting bigger, and we have to get bigger too — bigger with our ideas, our resources, and we need a really coordinated plan,” DeMarco said.

Some residents are uniting through community groups and new efforts to watch the streets and inform the authorities of suspicious activity.

“Skully” and “Bandit,” a couple who are members of a biker group known as Ryders Don’t Play, and who asked that their names be withheld for fear of gangs coming after their children, organized a Sept. 19 demonstration that entailed riding their bikes through the community as a show of unity.

They chanted “Save our children!” and “Stop the violence!”

Dozens of residents — bikers or not — joined them.

“Bandit,” 40, is running a community watch with bikers taking to the hamlet’s streets late at night. He said that more sets of eyes can help drive away illicit groups.

“We have to take back our neighborhood,” he said. “These parents should not have been burying their kids.”

Stephanie Spezia, 51, who has lived in Brentwood since 1974, hopes that more involvement from all residents restores the place she knew, where more neighbors talked to each another and their kids played together outside the houses or in area parks.

She thinks twice now before going out for a walk to the local convenience store, as she used to, but she has faith that residents together can make Brentwood “a community again.”

For too many years, she said, the gang problem has been ignored because it seemed to be a matter that only affected a subset of the population that joined those groups, but “the cops can’t be everywhere” and residents need to be aware and report any suspicious activity.

“People have always thought ‘There is a problem, there is a problem’ ” with gangs and crime, Spezia said, “but they may feel it’s not happening to me, and with these two little girls being murdered,” and the two boys’ bodies found later, “Everyone is saying, ‘Holy crap, it is in my backyard now.’ ”