A leader of the Salvadoran street gang MS-13 was sentenced Monday to life in prison for his role in the execution-style murders of a 19-year-old Hempstead woman and her 2-year-old son.

Before the sentencing, family members of Vanessa Argueta and her toddler, Diego Torres, spoke briefly in federal court in Central Islip Monday, asking that justice be served on Heriberto Martinez, 26, of Far Rockaway. U.S. District Judge Robert Joseph Bianco assured them "that justice will be done."

Minutes later, Bianco imposed three life terms on Martinez for the murders as well as several other violent felonies.

"This defendant should never be free again to commit crimes against other people," he said.

Martinez and co-defendant Carlos Ortega were convicted on March 21 on all 21 counts of an indictment charging them with racketeering, racketeering conspiracy, murder, assault with dangerous weapons and related firearms and conspiracy offenses. Ortega was sentenced on Nov. 21 to life in prison.

Authorities said Martinez authorized the 2010 shooting of Argueta because she had spoken with another gang member in a Hempstead bar -- a show of "disrespect" to MS-13 punishable by death under the gang's deadly rules, prosecutors said.

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Her son was killed, almost as an afterthought, because gang members thought he might seek revenge when he grew up, prosecutors said. The mother and son were taken to a wooded area in Central Islip where she was shot in the head and chest and the little boy was shot twice in the head.

Argueta's father, Jose Oscar Argueta, of Hempstead, thanked law enforcement and the justice system for prosecuting his daughter's killers.

"I want to thank God for giving me the strength to be here," he said in court, adding his daughter and grandson "will live in my thoughts for the rest of my life."

The toddler's father, Armando Torres, of Hempstead, said the killings were senseless. "They will always be in our hearts. They will always be here with us," Torres said.

Calling himself "a soldier" in MS-13, Martinez refused Monday in court to apologize to the families of the victims, because, he said, he is innocent. He said he was prosecuted because he refused to cooperate with law enforcement.

Martinez was previously convicted of killing Nestor Moreno, 23, a security guard at El Rancho Bar and Grill in Hempstead, on March 6, 2010, after a dispute over a bar bill. The handgun used to kill Moreno was the same one used to kill Argueta and her son, prosecutors said.

On March 17, 2010, Martinez killed Mario Quijada in Far Rockaway. Quijada, a fellow MS-13 member, was killed because of his reluctance to "put in work," or attack rival gang members, the jury found.