President Donald Trump at a speech about his effort against the MS-13 street gang, to a gathering of federal, state, and local law enforcement in Brentwood, New York, July 28, 2017. REUTERS/Jonathan Ernst

President Donald Trump, speaking in Brentwood, New York, on Friday, insisted that the MS-13 street gang, a target of his hardline immigration and anti-crime proposals, are turning US communities "into blood-stained killing fields."

Trump was speaking to law enforcement officers on Long Island, home of Suffolk County, which has seen 17 killings linked to the gang over the last 18 months.

Federal officials have charged suspects in only two of the homicides, including the September killing of two teenage girls from Brentwood.

In his speech, Trump mentioned that case and elaborated on other crimes attributed to the gang in graphic terms.

"They butchered those little girls. They kidnap. They extort. They rape, and they rob. They prey on children. They shouldn't be here," Trump said.

"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," he went on. "They're animals. 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."

A member of the Mara Salvatrucha street gang shows his tattooed arms during a break at a carpentry workshop of the prison in San Pedro Sula, Honduras, May 28, 2013. REUTERS/Jorge Cabrera

Trump has blamed his predecessor, former President Barack Obama, for allowing the gang to take root in the US, but it has actually been present in some form since the 1970s and 1980s, when immigrants from Central America — El Salvador in particular — fled conflicts there for the US, with many settling in Los Angeles.

An increase in deportations sent many of the gang's members back to Central America in the 1990s, returning them to countries to which many had no real connections or had never even been to.

In unstable post-conflict environments in El Salvador, Guatemala, and Honduras, the gang grew quickly. It has expanded into extortion, human smuggling, and firearms trafficking, among other crimes.

Alleged members of the 'Mara Salvatrucha' gang lay on the ground during a raid in Montreal neighbourhood, in the town of Mejicanos, a suburb of San Salvador, on March 20, 2012 © AFP/File Jose Cabezas

In the years since, it has continued to grow and spread to the US, where it is active among communities of Central Americans.

The Justice Department has estimated there are over 10,000 members in the US and that it is active in 40 states.

The gang's "cliques," or branches, in the US have mainly been involved in local-level crime — extortion, drug dealing, and theft.

But in places like Suffolk County and Montgomery County, Maryland, they are more active. The 17 homicides the gang is connected to in Suffolk County over the past year are 38% of the total.

Investigations of MS-13-linked killings in Montgomery County have jumped to seven each of the last two years after being about one in previous years. The gang has also grown more bold there, moving from targeting illicit businesses to extorting legitimate ones run by Central Americans.

Immigration and Customs Enforcement

Members of the Trump administration have compared the gang to Colombian or Italian mafias, and while MS-13 cliques have been linked to Mexican transnational drug cartels — working with the Sinaloa, Gulf, and Zetas cartels, according to the Drug Enforcement Administration — the gang's actual involvement in the drug trade appears to be limited, in some cases just to local markets.

Despite the focus on it, it's not clear that MS-13 is as large or as powerful as reputed.

Immigration and Customs Enforcement arrested 125,211 people in fiscal year 2015, with 322 MS-13 members among them, according to CNN. In fiscal year 2016, 114,434 people were arrested, with 429 MS-13 members among them. In the current fiscal year, which began in October 2016, 253 of the gang's members have been arrested.

Many have welcomed Trump's hardline on immigration and his focus on the gang. But both members and police have said it could backfire.