Authorities found 21-year-old Juan Elias Garcia in Nicaragua, where he was arrested.
Police say Garcia is a member of the MS-13 gang and is a suspect in the 2010 murder of his 19-year-old girlfriend and her toddler.
Garcia was detained Thursday near the U.S. Embassy in Managua and immediately turned over to the FBI. An FBI news release Friday said Juan Elias Garcia voluntarily returned to the United States to face charges
Garcia is the first MS-13 gang member to make the FBI's Most Wanted list.
Prosecutors allege the El Salvador native lured Vanessa Argueta and her 2-year-old son Diego Torres to the woods of Central Islip after promising to take her to dinner in February 2010, where two other gang members shot them to death.
He is the fourth person sought by federal authorities in the Long Island killings.
Argueta was accompanied by two other reputed MS-13 members who had received permission from the gang's leader to carry out the killing, FBI assistant director of the New York field office George Venizelos said. One of them shot her in the chest and head because of alleged threats made toward Garcia by rival gang members to whom she had ties, according to Venizelos.
Her son was also shot in the head twice - the second time after grasping at Garcia's leg - with the same .22 caliber handgun, he said.
"Garcia decided that penalty for disrespecting the gang was death," Venizelos said. "While crime and gang-related activity often goes hand in hand, the execution-style murders of Vanessa and her 2-year-old son is a new low even for MS-13."
Argueta's father, Oscar Argueta, said that when he heard about Garcia's arrest, he went to the cemetery.
"6 o'clock in the morning, I go and I bring flowers and I remember my daughter and my grandson," he said.
Last October, the Treasury Department designated MS-13, or Mara Salvatrucha, as an international criminal organization. It has a strong presence throughout the country, particularly in areas with substantial Salvadoran populations such as Southern California, the District of Columbia and northern Virginia. The gang also is allied with several Mexican drug cartels.
MS-13 is the largest and most violent street gang on Long Island, with more than a dozen chapters there, said Loretta Lynch, the U.S. Attorney for New York's Eastern District.
Garcia was indicted on seven counts that included murder, murder conspiracy and firearms charges last month, shortly after he turned 21, and lost federal juvenile protection status that had prevented authorities from naming him publicly, Lynch said.
Rene Mendez Mejia, who pleaded guilty to the murders, and Adalberto Ariel Guzman, who was found guilty at trial, are awaiting sentencing, Venizelos said. Gang leader Heriberto Martinez, who allegedly sanctioned the killings, was charged in connection with the murders and other crimes and has been sentenced to three life terms plus 60 years, he said.
(The Associated Press contributed to this report)