Leandra Feliz sat in the second row of a packed Bronx courtroom on Wednesday, staring down each of the 12 men charged with killing her 15-year-old son with machetes and knives.

“Asesino!” she shouted, Spanish for assassin, as the first two were led away in handcuffs after being arraigned on murder charges. A court officer warned her to be quiet, and Ms. Feliz stepped into a hallway of the Bronx Hall of Justice to compose herself.

Prosecutors said the defendants are all members of the Trinitarios, a highly organized and violent Dominican gang based in New York City. Each pleaded not guilty to murder, conspiracy and other charges in an indictment handed up by a grand jury for the killing of Lesandro Guzman-Feliz on June 20.

The indictment followed a monthlong investigation into Lesandro’s death. Captured on video that was shared widely across social media, the killing of a boyish-looking teenager at the hands of machete-wielding gang members has gripped the city, spurring outrage and support from local politicians, the police commissioner and celebrities like the rapper Cardi B and the Yankee pitcher C. C. Sabathia. The police said Lesandro’s killers mistook him for a member of a rival group.