The notorious gang has become a focal point of Trump’s immigration effort – but critics say innocent people are being swept up, causing greater upheaval and violence

“In Brentwood our community fears two things,” said Carlos Piovanetti as he thumbed through a bulging legal dossier strewn out on his desk. “There’s an immense fear of violent crime, and a tremendous fear of the government, of being picked up and deported.”

This small hamlet on Long Island, an hour’s drive from New York City, has recently found itself under the national spotlight following a series of gruesome local murders linked to the now notorious MS-13 gang. On Friday, Donald Trump will visit Brentwood, which has for decades attracted Hispanic migrants, including thousands from El Salvador. Here, he will deliver a speech in which he is expected to call for greater resources to crackdown on undocumented immigrants after claiming earlier in the week that his administration is “liberating” American cities from gang violence.

“We are throwing MS-13 the hell out of here so fast,” the president said in Ohio on Tuesday. “And, well, I will just tell you this, we’re not doing it in a politically correct fashion.”

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The gang has become the symbolic focal point of Trump’s harsh immigration regime. But while the violence is recognised as a scourge on the community, some in Brentwood question the administration’s approach to tackling the issue and point to Trump’s ignorance of the group’s origins and history.

Piovanetti, the managing attorney for the nonprofit Immigration Legal Services of Long Island, believed the legal dossier laid out on his desk, and the “garbage evidence” it contained, proved part of the point.

It related to the case of one 18-year-old undocumented El Salvadoran, who did not want to be named or interviewed, picked up as part of Operation Matador, a gang and immigration enforcement effort headed by the Department of Homeland Security (DHS) in partnership with local police. The teenager, who according to his attorneys was the drummer in a local church band and a hardworking high school student, had been arrested by local officers in April on a minor misdemeanor for failing to state his date of birth correctly. It was his first alleged criminal offense but he was apprehended by immigration authorities nonetheless, and was later labelled an MS-13 gang member.

In a memo to court seen by the Guardian, DHS claimed the boy had been observed in March last year “flashing an MS-13 hand sign” and had been seen on other occasions with other “confirmed MS-13 members” and had worn “clothing and accessories” indicative of MS-13. But the memo cited no sworn or documentary evidence for the claims and did not attribute a source for the allegations. The teenager was granted bail despite the memo’s request he be detained until a full trial.

Facebook Twitter Pinterest Immigration attorney Carlos Piovanetti. Photograph: Oliver Laughland/The Guardian

“How can they be presenting this garbage to a judge?” Piovanetti said. “It just breeds more distrust. I know families here who are too afraid to go to the supermarket out of fear of being detained.” The Guardian is aware of at least one more memo used in a similar case, citing unsourced non-criminal allegations of gang affiliation against young undocumented El Salvadorans arrested around Brentwood.

Suffolk County police department, the local agency who supplied DHS with the evidence of alleged gang ties, has been contacted for comment on the memo.

Such a strategy, said Piovanetti, who has practised immigration law in New York for decades, is unprecedented and indicative of the sweeping new powers DHS enjoys under the Trump administration.

But the operation has also seen some success.

At a White House press conference on Thursday, surrounded by images of MS-13 gang tattoos, acting Immigration and Customs Enforcement (Ice) director Thomas Homan told reporters that Operation Matador had “netted” over 100 gang members and affiliates since it began in May. Two juveniles apprehended in the operation had been implicated in a brutal MS-13 related quadruple homicide earlier in the year. The effort has also led to another gang member admitting involvement in the horrific murders of two teenage girls from Brentwood High School in September last year. Earlier this month, 17 MS-13 members were indicted on federal charges over their alleged role in a number of recent killings (local police attribute 27 murders to the gang since 2013).

There is little doubt that Trump’s strategy on MS-13 also serves up red meat to his base around the US. The president has frequently pointed to a declared crackdown on the organisation as evidence of an election promise to clamp down on criminal migrants, despite the fact that immigration arrests of those with no criminal history are surging around the US.

Critics point out the president has exhibited little to no grasp of the gang’s complex history, which is tangled in both US foreign and domestic policy. Instead, Trump has sought, incorrectly, to place blame on the previous administration for the recent spate of violence.

“The fact that he has described this gang as something that was created under the Obama administration is just one reflection of his absolute and complete ignorance when it comes to this group,” said Steven Dudley, co-director of InSight Crime, a foundation that researches organized crime. “So until there is a better understanding of how we got to where we are, then the gang is just going to persist.”

Facebook Twitter Pinterest El Salvadoran vice consul Julio Cesar Martinez Pineda. Photograph: Oliver Laughland/The Guardian

MS-13, a factional group with no clear leadership structure, was founded in Los Angeles in the 1980s following a wave of migration from Central America during bitter civil wars in the region. Under the Reagan administration, migrants from El Salvador and Guatemala were offered little protection in America, which backed the regimes from which they fled and characterised most as “economic migrants”, overwhelmingly denying asylum. Of the thousands that did settle, some were drawn into gangs as a means of survival.

The escalation of violence and crime continued through the following decade leading to a series of hardline criminal justice reforms under the Clinton administration, which saw many gang members incarcerated and then deported along with others convicted of lesser crimes. These deportation efforts ratcheted up even further under George W Bush, essentially funneling thousands more people back to greater upheaval and violence and perpetuating another cycle of displacement.

Dudley warned that Trump’s dragnet approach to immigration enforcement, which allows Ice agents to target essentially anyone without paperwork, could lead to “going full circle again”. Like Piovanetti, he warned that implementing such broad enforcement priorities, would alienate undocumented communities making it less likely they would report information on serious criminality.

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Such distrust was palpable at the El Salvador consulate in Brentwood on Thursday as dozens of people waited to see officials, many to collect passports or seek consular advice. None of those present were willing to be interviewed using their full names, many citing fears they were undocumented, but frequently people expressed resentment over Trump’s visit.

Even the El Salvadoran vice consul, Julio Cesar Martinez Pineda, told the Guardian through a translator: “Many people have fear here after the election,” before going on to laud the recent US-Salvadoran efforts to combat MS-13.

Another of Piovanetti’s clients, a mother whose son is also believed to have been swept up in Operation Matador, claimed her 22-year-old had no gang affiliation. He had been arrested for marijuana possession in April and, after pleading guilty, was arrested by Ice.

The mother, also undocumented and unwilling to give her name for publication, had one message for Trump: “Yes there are some bad people here in Brentwood, but it is not all of us.

“If my son is sent back to El Salvador, they [a gang] will kill him. They will say: ‘Either join us, or you are killed’.”