Members of the notorious MS-13 have been charged with murder. Reuters

Thirteen members of the violent Mara Salvatrucha gang (MS-13) have been indicted on seven murder charges, including the slaying of three teenagers killed in Brentwood.

Four of the gang members arrested were charged with the murders of high school best friends 16-year-old Kayla Cuevas and 15-year-old Nisa Mickens, who were beaten to death in Brentwood in September 2016, and four others who were charged with the death of Jose Peña-Hernandez, 18.

Peña-Hernandez is believed to have been a member of MS-13. He is thought to have been killed by the gang because they believed he had violated their rules.

In total, the 13 gang members were charged with 41 counts in connection with seven murders, including charges of attempted murder and assault.

"The day Nisa Mickens and Kayla Cuevas were heinously murdered, the Suffolk County Police Department made a commitment to their families and to the residents of Brentwood that justice would be served. Today, in collaboration with the FBI and the United States Attorney's Office, we have delivered on that promise," Suffolk County Police Commissioner Tim Sini told Fox News.

At the time of the teenagers' deaths, Sini said "gang involvement" was suspected, although he did not elaborate further.

"The only people in Brentwood who have anything to fear are the criminals," Sini said at the time, in comments carried by The New York Times. "That's because there is a tsunami of law enforcement officers at their doorsteps."

More than 125 members of the gang have been arrested since the deaths of the teenagers last year, while MS-13 is believed to have been responsible for at least 30 killings on Long Island over the past seven years.

The killing of the teenagers was last year mentioned by US President Donald Trump, who circulated the discussion around crimes committed by undocumented immigrants.

"They come from Central America. They're tougher than any people you've ever met," he said last December in comments carried by the Associated Press. "They're killing and raping everybody out there. They're illegal. And they are finished."