Thirteen MS-13 gang members were charged Thursday for committing more than a half dozen murders, including the 2016 slayings of three Brentwood teenagers, authorities said.

The brutal deaths of 15-year-old Nisa Mickens and 16-year-old Kayla Cuevas last September sent shockwaves across the community and prompted pledges from law enforcement that the killers would be brought to justice.

Five months later, they were, authorities said.

The 13 suspected gang members include 10 who are undocumented, a green card holder, and a pair of U.S. citizens, authorities said.

Charged in their alleged roles in the murders of Mickens and Cuevas were Selvin Chavez, Enrique Portillo, both 19 and from Brentwood, and two juveniles who were not identified.

The foursome were riding in a car on Sept. 13 and looking for rival gang members when they came upon the two girls, authorities said. They dialed the leaders of their MS-13 clique, brothers Alexi and Jairo Saenz, who allegedly endorsed the slayings. Cuevas ran for her life but was bludgeoned and stabbed after her attackers caught up to her inside a fenced-in backyard. The Saenz brothers were also charged.

“Kayla Cuevas was marked for death because she had feuded at school and over social media with various members of MS-13,” Robert Capers, U.S. Attorney for the Eastern District of New York, said at a press conference at federal court in Central Islip on Thursday. “And Nisa Mickens was simply at the wrong place at the wrong time.”

Mickens’ lifeless body was “almost unrecognizable” when it was discovered, Capers said.

The gang’s dispute was with Cuevas, who was involved in a series of feuds with MS-13 members months prior as well as another altercation at Brentwood High School that escalated the vendetta, authorities said.

What rational adults may consider innocuous bickering, MS-13 gang members interpreted as death sentences, with their enforcers serving as executioners, authorities said.

“Gangs viciously defend their territories and ideals, allowing themselves to engage in acts of violence against rivals, outsiders and non-members,” said William F. Sweeney, FBI assistant director-in-charge. “They retaliate against anyone who dares to close in on their territory—a territory that is often imagined and self-defined.”

The spate of murders last year also claimed 18-year-old Jose Pena-Hernandez, an MS-13 gang member who authorities said was killed because he was suspected of being an informant and violating supposed gang rules.

Pena, who also attended Brentwood High School, was lured into a car and driven to the abandoned Pilgrim Psychiatric Center in June, one of the gang’s hangouts. Gang members from the group’s Freeport affiliate took turns stabbing and slashing Pena with a machete. His battered body was left in a wooded area to decompose. Pena’s skeletal remains were found in October. He was reported missing on June 13; by then he was already dead.

Capers declined to say whether Pena was cooperating with law enforcement, but noted that local gang members clearly believed he was. Among those charged in Pena’s death were 18-year-old German Cruz of Brentwood and 24-year-old Reynaldo Lopez-Alvarado.

Capers said the investigation is ongoing.

Suffolk County Police Commissioner Tim Sini, in perhaps the most high-profile case since he was named to the position, said justice, as promised, had been served.

“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,” Sini said. “Today, in collaboration with the FBI and the United States Attorney’s Office, we have deliver on that promise.”

The murder charges were part of a 41-count superseding indictment unsealed Thursday. The other charges include racketeering, attempted murders, assaults, obstruction of justice and more.

The sweeping indictment covers seven murders from 2013 through 2016, including the slayings of Pena, Cuevas and Mickens, all Brentwood High School students. Thursday’s indictment replaces one filed last year that covered four murders, all of which are included in the new document. Three suspects covered under the original indictment have previously appeared in court and were ordered detained, pending trial.

Murder charges included in the new charging document date back to the 2013 slayings of Derrick Mayes and Keenan Russell, and the murders of Jose Lainez-Murcia and Jonathan Cardona-Hernandez, in 2014 and 2015, respectively.

Five suspects were arrested Thursday morning and were scheduled to be arraigned in the afternoon. Two have yet to be detained, but their identities were not released.

The investigation was carried out by the Long Island Gang Task Force, which encompasses federal authorities and local and state agencies, including Nassau and Suffolk police.

Capers declined on Thursday to elaborate on investigative tools that led police to track down those responsible for the slayings in five months time. But a detention letter submitted to the court noted that the government’s evidence includes testimony of eyewitnesses and surviving witnesses and testimony from former MS-13 members who have pleaded guilty to various crimes. Evidence gleaned from those witnesses has “provided a wealth of information regarding the defendants’ participation in the charged offenses and other violent crimes on behalf of MS-13,” states the detention letter in which prosecutors also argue that the suspects be denied bail.

The unit has been pursuing MS-13 associates for years and has brought charges against dozens of alleged gang members, including 35 murder indictments since 2010. MS-13 is regarded as the most dangerous street gang on LI, with roots in El Salvador and Honduras.

Capers said he hopes the arrests convince Long Islanders that law enforcement is up to the task of combating gang violence.

“It’s our sincere hope that these arrests will send a message to all the communities on Long Island, many of which are populated by hard working and law-abiding people from all walks of life, that we in law enforcement stand by you and will continue to protect and serve you and do the best that we can,” he said.

Related: 6th Brentwood Gang Violence Victim Found Dead in 5 Weeks

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