On West Greenwich Avenue in Roosevelt, Long Island, the oak trees were just thick enough to hide Angel Soler’s mutilated body.

The 15-year-old boy had been stabbed to death with a machete and dumped in a wooded area bordering the Southern State Parkway by members of the vicious El Salvadoran gang MS-13.

His family said he had been threatened by the gang in July, although it is still unclear why. Soler wasn’t a suspected member of MS-13 — he had actually fled his native Honduras more than four years earlier to escape gang violence. He went missing shortly after receiving threatening texts, and was found dead Oct. 19.

“You’re not safe anyplace,” Roosevelt resident Sybil Greenidge, 76, recently lamented to The Post, standing a few dozen feet from the area in Nassau County where Soler’s body was found.

The mouth of the woods is in her back yard, at the end of a dead-end street, just beyond her son’s basketball hoop. MS-13 members used the thick, leafy area — and the roar of the parkway — as cover while they brutally murdered Soler. Greenidge never even heard a scream.

While MS-13 has been operating in neighboring Suffolk County for the past decade, its increasing infiltration of Nassau is alarming authorities — and terrifying residents more used to worrying about the traffic on the Long Island Expressway than gang warfare.

“Thank God I haven’t been killed,” Greenidge said, standing in the doorway of her home on a serene, tree-lined block of manicured lawns and two-story brick-and-stone houses. “It’s crazy something like this could happen so close to your house.”

MS-13’s motto is “Murder, rape, control.’’

Authorities consider it the world’s most dangerous street gang at the moment, and its heavily tattooed, machete-wielding members easily live up to the hype.

The gang was born in Los Angeles in the 1980s in the wake of deadly civil wars wracking the three countries forming the so-called “Northern Triangle’’ at the top of Central America: El Salvador, Guatemala and Honduras.

Refugees from those countries fled to the United States, landing mostly in poor LA neighborhoods, leaving them vulnerable to Mexican street gangs already in power. The refugees banded together to fight back, taking cues from the Mexican gangs while forming their own version of a ruthless organization.

The new gang of street terrorists dubbed themselves Mara Salvatrucha 13, or MS-13 for short. The name is believed to be a combination of the Spanish word mara, or “gang,’’ Salva for Salvador and trucha, street slang for staying vigilant. The number 13 supposedly refers to M’s place in the alphabet — an homage to Mexico, the home country of the gangs that gave it its start.

About three decades after first hitting the US, the gang has infiltrated more than 40 states with 10,000-plus known members, according to FBI estimates. Their numbers in New York are murky, but one thing is certain: Long Island has become one of the gang’s major East Coast strongholds after Washington, DC, and its surrounding areas, authorities say.

The gang follows work opportunities, officials say: Where there are wealthy areas in need of cheap immigrant workers, you will find MS-13.

The gang has developed a grip especially in Suffolk County in the past 10 years, mostly in the Hispanic neighborhoods of Brentwood and Central Islip. Two sets of slayings tied to the gang in those towns have garnered national attention — including from President Trump and Attorney General Jeff Sessions, both of whom visited the county last spring to personally vow to eradicate the gang.

First, there were the double murders of Kayla Cuevas, 16, and her friend Nisa Mickens, 15, who were hacked to death by MS-13 members in Brentwood in 2016 after getting into a schoolyard fight with one of the gang’s thugs.

Then came the quadruple homicide of Justin Llivicura, Michael Banegas, Jorge Tigre and Jefferson Villalobos in a park in Central Islip this year.

The only working motive? The victims somehow “disrespected’’ MS-13.

But MS-13’s presence is slowly but surely spreading to adjoining Nassau County, following new work opportunities.

“We live in this nice neighborhood, and this is not what you would expect,” said a mom who lives with her kids in the Gates of Woodbury community on Long Island’s tony North Shore, where houses go for $1 million-plus.

Her home was recently vandalized by suspected members of MS-13.

“It freaks me out . . . I felt violated,’’ she said. “I sleep right there, and they were right [below me]. We’re upgrading our whole alarm system.”

Since last year, MS-13 has been responsible for at least nearly 30 deaths across Long Island, authorities say. Dozens more suspected victims remain missing.

MS-13 is not like other gangs, the head of Nassau’s gang unit, Detective Sgt. Michael Marino, told The Post.

“They’re more organized, more sophisticated than you think,” he said. “They have a very distinct structure, a very strong punishment scale for not following certain rules within their gang.’’

The gang also is very ritualistic. For example, new members are initiated with a 13-second beating, and higher-ups divide their turf into 13 units to oversee.

Marino said MS-13’s bosses don’t care about making money like other gangs do — many of their members work as day laborers in places such as Home Depot and restaurants. One gang member doubled as the caretaker of the sprinkler system of a multimillion-dollar mansion on the North Shore.

Instead, the sadistic gang’s main interest is power.

“In other words, ‘We are the gang, we are in control, this is our territory, everyone will follow our rules,’ ” Marino explained. “The more I learn about MS-13, the more I felt like I underestimated their organization.”

The gang’s presence remained relatively steady on Long Island until around 2014 and 2015, when it started to spike, authorities said. Marino attributed that mostly to an influx of “unaccompanied alien children,” or UACs, into the area — about 10,000 since 2014. UACs are minors who cross into the US alone without parents or guardians.

Marino estimated that 90 to 95 percent of UACs are legitimately trying to escape the poor conditions and violence taking over the Northern Triangle, which now has some of the highest murder rates in the world.

Marino said the gang, whose members are typically between 15 and 25 years old, uses the UAC “pipeline” to get current members here from Central America, as well as to boost its ranks with newbies.

Even if the kids aren’t gang members when they come into the country, they experience tremendous pressure to join once they arrive, he said. They are primarily recruited while in the school system, Marino said.

“You’re taking a kid without parents . . . They don’t speak English . . . In school, they need ESL,” or English-as-a-second-language classes, which means they’re around the same group of kids all day long, Marino explained.

‘They lure them in either under the guise of girls or smoking marijuana. And they befriend a lot of them, too. They’ll go in the woods 10 times and smoke marijuana. On the 11th time, you get whacked.’

“Say [MS-13 has] a couple gang members or a couple bullies in there. They’re pressuring [the new kids] all day in the same class . . . recruiting for the gang or the rival gang. So [the UACs] are put in a very difficult situation . . . They are very high-risk to be recruited into the gang,’’ he said.

Marino said during a recent MS-13 sweep on Long Island, 22 percent of the people arrested were UACs. “That’s a statistically significant number,” he said.

When Soler’s body was found on Oct. 19, it was one of three corpses recovered within five miles of each other in the span of nine days.

All three victims — and three others whose bodies were recovered in Nassau earlier in the year — are believed to have been killed by MS-13.

After Soler’s body was discovered, Javier Castillo, 16, was found five days later in Freeport’s Cow Meadow Park. The park sits near Freeport’s waterfront, across from a development boasting “luxury, oceanfront condos.”

“The body was buried off the nature trail,” a park ranger told The Post after the discovery. “The cops aren’t coming out here as much as they should.”

Castillo and Soler’s murders followed MS-13’s “protocol” perfectly.

“It’s always the same: in the woods, lured in by the same mechanisms . . . It’s a very similar script,” Marino said. “They lure them in either under the guise of girls or smoking marijuana.

“And they befriend a lot of them, too. They’ll go in the woods 10 times and smoke marijuana. On the 11th time, you get whacked.”

Freeport is considered to be one of Nassau’s MS-13 hot spots.

On Sept. 15, Miguel Ayala-Hernandez, 18, was murdered in front of Freeport High School in what investigators said was an act of gang retaliation. Police said Ayala-Hernandez embarrassed a gang member in front of his crew by chasing him. He was shot nine times and found bleeding to death in the middle of a residential street.

Vanessa Interian, a 16-year-old junior at the school, said she knows gang members are around.

“You have to know who you hang out with and who they associate with,” Interian said from her mother’s car during pickup on a recent afternoon.

“I’m more aware now [after the murder].”

She recounted how gang members swarmed one student and jumped him just outside of school grounds soon after Ayala-Hernandez’s slaying.

“They just beat the boy, like 10 kids on him. I couldn’t even watch it,” Interian said. “I was like, this is crazy. I wanted to do something about it, but I can’t. You can’t do anything.” Her mother, Rosa, said it hasn’t been easy knowing gang members are at the school. “I’m scared, but there’s nothing we can do,” the 42-year-old mom said.

Another mom, who didn’t want to give her name, said she doesn’t let her 17-year-old son walk home alone anymore.

“These crimes are heinous . . . Who dismembers people and buries them?” she said. “Those [gang members] are animals; those aren’t regular children.”

Long Island police say there is no reason for the public to panic about MS-13. But it is hard for many residents to shake the effect the gang is having on their communities.

On a recent early Friday evening at the Central Islip Recreation Village Park in Suffolk, where the four young men were found, the site was mostly empty.

“It’s still scary coming here,” a young mom said while her two children ran around. She looked around uneasily as she spoke, her eyes darting from the trees and back to her children.

When her daughter got closer, she asked The Post to leave. “I don’t really want to talk about it. She understands what we’re saying,’’ the woman explained.

Survi Mukerjae, 35, who walked her dog past a memorial for the four slain youngsters, said, “I’m trying not to be [nervous]. I feel like when you stop doing the things that you really enjoy, that’s when it becomes a problem.”

In Nassau, husband and wife Nancy and George Tvelia, both 71, said they used to visit Cow Meadow Park at least twice a week for bird-watching. That was before Castillo’s corpse was found. “We haven’t been here ever since they mentioned the body,” George said. “Sometimes, we’d walk the trail, and we didn’t know if there were going to be gang members there or not.”

Bob Smith, 72, still likes to walk the track — even though the area “has gotten massively worse.

“Now, all of a sudden, you hear shots fired every night around here . . . They’re savages,” said Smith, a retired Freeport cop.

“This is a beautiful area, all maintained houses — except when the sun goes down, the garbage comes out, and that’s when it starts.”