CENTRAL ISLIP, N.Y. — The four young men sat nonchalantly in a federal courthouse here on Thursday afternoon, barely raising their eyebrows when a judge said they could face the death penalty for their roles in the killings of two teenage girls in September.

Instead, one of the four, Jairo Saenz, a leader of the local clique of a transnational gang with Salvadoran roots, grinned, swiveled in his chair and hardly looked at the crying relatives from El Salvador in the back of the courtroom.

Mr. Saenz, 19, was one of five people arrested Thursday morning in Suffolk County on Long Island as part of a federal sweep that targeted 13 adult members of the gang, MS-13. An additional three people were charged as juveniles, and their records were sealed. As of Thursday evening, 14 people were in custody, with two yet to be arrested.

In a midday news conference at the federal courthouse here, Robert L. Capers, the United States attorney for the Eastern District of New York, announced the indictments and the arrests made by the Federal Bureau of Investigation’s Long Island gang task force.