The president spoke Friday afternoon at Suffolk County Community College in Brentwood, close to where MS-13 has committed a string of gruesome murders, including the massacre of four young men in April in a Central Islip park.
Last year, authorities said two teen girls in Brentwood were murdered by gang members as well.
"Few communities have suffered worse at the hands of these MS-13 thugs than the people of Long Island." Trump said. "Our hearts and our nation grieve for the victims and their families."
"They butchered those little girls," he added.
The mother of one of the victims, Kayla Cuevas, attended the speech and told CBS2's Tony Aiello the words were difficult to hear, but necessary.
"It was rough. It's rough, but it's real," Evelyn Rodriguez said. "This is what's happening, and I'm glad that he does recognize that."
Trump continued his tough talk on immigration and urged Congress to dedicate more funding to border enforcement and faster deportations in a speech in front of law enforcement officers and the family members of crime victims.
He called for hiring an additional 10,000 immigration enforcement agents and hundreds of additional immigration court prosecutors and judges, Aiello reported. He promised to stem the flow of unaccompanied minors entering the country illegally.
Trump also told law enforcement officers they do rough work and he doesn't mind when they act roughly, especially when putting suspects into police cruisers.
"Like when you guys put somebody in the car and you're protecting their head, the way you put the hand over, like don't hit their head -- they just killed somebody, don't hit their head. I say you can take the hand away, OK?" he said.
Some officers laughed and cheered.
"I think they realize that was in jest," Suffolk County Police Deputy Inspector Milagros Soto said. "They know what our rules and procedures are, and we would not accept that."
Chris Dunn, of the New York Civil Liberties Union, told WCBS 880's Peter Haskell that he thinks it goes beyond campaign-style hyperbole.
"It's truly shocking to hear in encouraging cops to be roughing up people who are under arrest and in handcuffs," he said. "This is a clear message to police officers to engage in brutality, and everybody should be rejecting that.
The president also seemed to poke at a certain big city mayor, Aiello reported.
"I've met police that are great police that aren't allowed to do their job, because they have a pathetic mayor, or a mayor that doesn't know what's going on," he said. "Were you applauding for someone in particular? It's sad, it's sad."
Mayor Bill de Blasio certainly took it personally. His campaign sent out an email, saying the president is "taking aim at the mayor" and asking for online donations of $3 or more. The email also defended the mayor's record on crime and called the president's speech "rambling and incoherent."
"President Trump knows 'Blue Lives Matter,'" said Rep. Peter King in his introduction of Trump. "MS-13 is going to be destroyed and we're not going to worry about political correctness."
Trump has made cracking down on MS-13, also called Mara Salvatrucha, a top priority of his administration.
"Together we're going to restore safety to our streets and peace to our communities and we're going to destroy the vile criminal cartel MS-13 and many other gangs," said Trump.
The gang, which is believed to have originated in immigrant communities in Los Angeles in the 1980s and then entrenched itself in Central America when its leaders were deported, is infamous for its violent tactics, including torturing victims and hacking them with machetes.
"They kidnap, they extort, they rape and they rob. They prey on children. They shouldn't be here. They stomp on their victims, they beat them with clubs, they slash them with machetes and they stab them with knives. They have transformed peaceful parks and beautiful quiet neighborhoods into blood-stained killing fields. They're animals," Trump said. "We cannot tolerate as a society the spilling of innocent, young, wonderful, vibrant, people. Sons and daughters, even husbands and wives. We cannot accept this violence one day more, can't do it and we're not going to do it."
Robert Hur, a top official at the Justice Department, said the gang's motto is "mata, viola, controla," which means "kill, rape and control."
"They seek to live up to this motto by truly shocking acts of violence designed to instill fear," Hur said. "Vicious machete attacks, execution-style gunshots, gang rape and human trafficking."
Its recruits are middle- and high-school students, predominantly in immigrant communities, who are said to risk violent retribution if they leave. Authorities estimate the group has tens of thousands of members across several Central American countries and many U.S. states.
Trump's Justice and Homeland Security departments have made targeting the gang a top priority. Attorney General Jeff Sessions has directed his department's law enforcement agencies and federal prosecutors across the country to prioritize their prosecution, as directed by an executive order Trump signed in February, among other measures.
"One by one we're liberating our American towns," Trump said. "One by one we are indeed freeing up these great American towns and cities that are under siege from gang violence."
"I have a simple message for every gang member and criminal alien that are threatening so violently our people: We will find you, we will arrest you, we will jail you and we will deport you," Trump added.
Since the beginning of January, the Department of Homeland Security's investigative unit has arrested 3,311 gang members across the country in a number of targeted operations, said Tom Homan, the acting director of U.S. Immigration and Customs Enforcement. The agency could not provide numbers for a similar timeframe in 2016.
"Targeting, arresting and removing members of violent street gangs such as MS-13 sends a clear message to criminal enterprises around the world: You are not welcome in the United States. You will find no harbor here," Homan said.
Trump's trip comes as Sessions, who was on Long Island to talk Ms-13 this past spring, is visiting El Salvador as part of a mission to increase international cooperation against the gang. Sessions met Thursday with his Salvadoran counterpart and members of an international anti-gang task force.
King, who represents Brentwood and traveled with the president from Washington, said that Trump's appearance would send a signal to communities that have been shaken by the violent killings.
"It's absolutely devastating. And almost all of these killings have occurred in my district, within 20 minutes of my home," he said.
King said the gang is responsible for 17 murders between January 2016 and April 2017 in his district — but that the impact on largely immigrant communities has been larger because of the way the gang kills. In addition to torturing victims, King said, members have also sent video of gruesome crime scenes to their victims' loved ones.
The Trump administration blames the gang's recent resurgence in certain areas on illegal immigration and believes policies like building a wall along the southern border and cracking down on so-called "sanctuary cities" will eradicate the problem.
"Failure to enforce our immigration laws had predictable results: drugs, gangs and violence but that's all changing now. Under the Trump administration, America's once more a nation of laws and once again a nation that stands up for our law enforcement officers," Trump said. "From now on we're going to enforce our laws, protect our borders and support our police like our police have never been supported before."
Critics see the focus misplaced and argue resources could be better spent on other enforcement efforts.
Meanwhile, there were several roads impacted by on and off closures during the president's visit.
"Suffice it to say, when you have the President of the United States coming into a suburban or urban area, you can expect major traffic implications," Suffolk County Police Commissioner Timothy Sini said. "You should plan accordingly."
Among those affected were the Long Island Expressway, Northern State Parkway, Sagtikos Parkway, Wicks Road and Crooked Hill Road near the Suffolk Community College Brentwood campus.
Click here to check current traffic conditions.
Late Friday night, the White House said Trump will sign a bill imposing new sanctions on Russia for meddling in the 2016 presidential election. It also blocks him from reversing the sanctions and targets Iran and North Korea, as well.
(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)
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