Attorney General Jeff Sessions pledged “to demolish” an MS-13 street gang he said is bent on killing, raping and destroying, during a visit to Suffolk County on Friday, as he assured law enforcement officials fighting the threat on Long Island that he and President Donald Trump “have your back.”

Sessions offered specifics about the scope and reach of MS-13, laid out elements of the government’s battle plan, and defended the president’s strict immigration policy as one that “ends the rampant . . . illegality” while leaving flexibility for investigators, in a speech before police commissioners, sheriffs and other local officials at the Central Islip federal courthouse.

As he spoke, about 100 demonstrators critical of Trump’s immigration policies gathered outside, speaking out against strict enforcement that makes immigrants vulnerable to deportation, even those that assist police. “Say it loud, say it clear, Sessions you’re not welcome here,” some shouted.

Sessions, however, remained unapologetic.

“I have a message for the gangs that are targeting our young people: We are targeting you. We are coming after you,” Sessions said.

The Justice Department will have “zero tolerance” for gangs, he insisted.

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“If you are a gang member, know this: We will find you. We will devastate your networks,” Sessions said. “We will starve your revenue sources, deplete your ranks and seize your profits. We will not concede a single block or street corner to your vicious tactics.”

Authorities say the MS-13 gang is responsible for a series of savage slayings in Suffolk’s Brentwood and Central Islip hamlets, with the body count of victims in incidents linked or thought to be related to gangs rising to 11 people in the last year. Those victims have been young Latinos and blacks, many of school age.

Sessions said Trump “explicitly talked to me about the murders you’ve seen here, the young people we lost,” adding that in “the Trump era, and you can be confident, law enforcement and leadership, we have your back.”

Trump singled out MS-13 himself, speaking to gun owners at the NRA Leadership Forum in Atlanta on Friday. “You know about MS-13? It’s not pleasant for them anymore, folks. It’s not pleasant for them anymore. That’s a bad group . . . Get them the hell out of here, right? Get them out.”

Protesters remained skeptical about the administration’s focus, particularly as they said the issue has been entangled with the debate over illegal immigration, which they fear will curtail cooperation between local police and affected communities.

Gabriela Castillo, coordinator of the Long Island Civic Engagement Table, which promotes civic participation among communities of color, said Sessions was not here to help make Brentwood and Central Islip safer, as he claims.

“I don’t think this has anything to do with public safety,” she said. “This is a convenient opportunity for him to make political talking points and to promote an anti-immigrant agenda.”

A small group of counter-demonstrators also gathered.

Sessions dismissed advocates’ criticism as “an exaggerated argument,” though he refused to offer guarantees to people in the country illegally that they wouldn’t be deported in exchange for cooperation.

Rep. Peter King (R-Seaford), who invited Sessions in the wake of the brutal slaying earlier this month of four young men whose bodies were found in a park a couple of miles from the courthouse, said those outside “should be out there on their knees, thanking” Sessions for paying attention to the gang issue, “not protesting.”

Immigrants who want to cooperate can still “call in the 911 anonymously,” Sessions said, adding that to his knowledge there were no cases of seeking witnesses to deport them.

Sessions met privately with law enforcement officials — including police commissioners and sheriffs from Suffolk and Nassau, the Hempstead village police chief and representatives from the state police, and the Suffolk probation department. Attendees characterized it as a friendly exchange where Sessions mostly listened to their assessments of the local gang situation and registered their concerns and requests for more federal prosecutors in this region.

“I think you will see some more personnel . . . some more focused efforts coming from Justice” as a result of the meeting with locals, said King. “It’s hard to quantify but it means a lot” to have the federal department’s backing.

Suffolk Police Commissioner Timothy Sini called it “a very positive meeting” in which he brought up a $500,000 grant that the police department and the federal prosecutors have applied for to “help offset some of our patrolling” costs. He added that “certainly we had law enforcement officials in that room who were very receptive of our mission to eradicate MS-13 and very supportive of providing necessary resources to get that job done and that’s a great thing for Suffolk County.”

Sessions also received the families of two suspected victims of the gang, teenage girls whose bodies were found half a year ago in a Brentwood neighborhood, following a brutal assault that law enforcement sources said involved machetes and baseball bats. Other relatives of victims of gang violence complained, though, that they had not been invited.

Overall, Sessions’ message was consistent with previous statements from him and the administration, which connect the gang problem to larger plans involving immigration controls, tight borders and plans to crack down on crime networks.

He said the United States must crack down on illegal immigration, in particular across the U.S.-Mexico border.

“We cannot continue with this transporting across our border illegal immigrants who have not been properly vetted and actually are part of criminal organizations,” he said. “We’ve got to dismantle these human smuggling networks that have become pretty effective in bringing people into the country illegally.”

Sessions added that the problem is not unique to Long Island. MS-13 now has 10,000 members in at least 40 states, he said. The gang, he added, has about 30,000 members outside the United States, with its international headquarters in effect based in the prison system of El Salvador.

“Transnational criminal organizations represent one of the gravest threats to American public safety today,” he said. “They enrich themselves by peddling poison in our communities, trafficking children for prostitution, inflicting horrific violence in our neighborhoods where they operate, and shipping the ill-gotten wealth back to their home criminal enterprise.”

Sessions’ visit comes as politicians and law enforcement officials have declared war against gangs operating in the region and specifically against MS-13. The gang, which is said to have originated in Los Angeles and continued to grow out of El Salvador’s civil war in the 1980s, is known for vicious assaults that, for the most part, target perceived rivals in immigrant communities.

With David Olson, Andrew Smith and Bart Jones