Three alleged members of the MS-13 street gang have been charged in the murders of four young men who were found dead in a park in Central Islip in April, according to an indictment that was quietly unsealed Monday.

The three suspects were arraigned Thursday in U.S. District Court on Long Island, according to the attorney for one of the men. Their next court appearance is likely in September.

The indictment identified the defendants in the Long Island case as Alexis Hernandez, Santis Leonel Ortiz-Flores and Omar Antonio Villata. Attorneys for Ortiz-Flores and Hernandez declined to comment; the name of Villata's attorney was not immediately available. All three suspects identified in the indictment are in custody.

The name of a fourth suspect was blacked out in court papers; it was not immediately clear if that person has been arrested.

The deaths of the four men focused national attention on MS-13, spurring Congressional hearings, tweets from the White House and a visit by Attorney General Jeff Sessions, the arrests flew under the radar. The mother of one of the victims said she and the other families only learned of the arrests yesterday, after the indictment was made public. In contrast, in March authorities announced the arrests of more than a dozen alleged gang members with a highly-publicized press conference.

A federal prosecutor on Monday declined to comment, saying any information released about the case would jeopardize an ongoing investigation. An FBI spokeswoman also declined to comment. 

MS-13, also called Mara Salvatrucha, is believed to have been founded as a neighborhood street gang in Los Angeles in the mid-1980s by immigrants fleeing a civil war in El Salvador. It grew after some members were deported to El Salvador, helping turn that country into one of the most violent places in the world.

19 killings have been blamed on MS-13 since January 2016.