The Suffolk County Grand Jury late yesterday returned indictments charging five Dorchester men in connection with the Sept. 25, 2011 shooting death of a 16-year-old boy and the non-fatal shooting of his 14-year-old friend on Geneva Avenue, District Attorney Daniel F. Conley said.

The indictments each charge SHELDON MATTIS (D.O.B. 1/22/93) and NYASANI WATT (D.O.B. 10/5/93), with first-degree murder, armed assault with intent to murder, assault and battery with a dangerous weapon causing serious bodily injury, unlawful possession of a firearm, and carrying a loaded firearm. Watt was additionally indicted on a second charge of unlawful possession of a firearm, unlawfully carrying a loaded firearm, and possession of a large capacity feeding device for a handgun he had on him at the time of his arrest on November 3.

Also indicted were: MATTHEW MITTON (D.O.B. 9/8/90) on four counts of witness intimidation from an incident that occurred in Dorchester on November 17; MICHAEL TAVARES (D.O.B. 12/1/87) on three counts of witness intimidation from an incident that occurred in Dorchester on November 28; and RANDALL VARISTE-SCOTT (D.O.B. 11/8/93) on a single count of perjury for statements he made on November 10 in the Grand Jury.

Mattis and Watt were arraigned in Dorchester District Court on November 3; Mitton, Tavares, and Variste-Scott were indicted as a result of evidence developed in the course of the Grand Jury investigation. All five men will be arraigned in Suffolk Superior Court at a later date.

At the arraignment of Mattis and Watt, Assistant District Attorney Mark Hallal, chief of Conley’s Senior Trial Unit, said that the two defendants were associated with a gang known as Flatline, based on Levant Street in Dorchester. On the afternoon of Sept. 25, people associated with that gang saw the surviving victim standing outside a convenience store right around the corner on Geneva Avenue.

Prosecutors believe that Mattis approached that boy to determine whether he was from a rival gang. After a brief discussion with the 14-year-old, prosecutors allege that Mattis went back to Levant Street and told his associates that the boy was a rival gang member.

The younger boy then met up with his friend, Jaivon Blake, the 16-year-old victim.

Watt allegedly armed himself with a .40 caliber semiautomatic handgun and rode on a bicycle to the intersection of Levant Street and Geneva Avenue, where he met up with Mattis.

Mattis allegedly helped Watt conceal the handgun by pulling Watt’s sweatshirt down over it.

Prosecutors believe that it was then that Watt rode his bike toward the two boys and fired multiple shots, striking Blake in the back and the younger teen in the neck.

In the days and weeks that followed, Boston Police homicide detectives developed evidence sufficient to charge Mattis and Watt. Mitton and Tavares have since been taken into custody; Variste-Scott will be summonsed for arraignment in Superior Court.

Attorney Harold Hakala represents Mattis, and attorney Willie Davis represents Watt. Variste-Scott is represented by attorney Neil Crowley. Mitton and Tavares will have attorneys appointed to them at arraignment in Suffolk Superior Court.