Juan Elias Garcia, an alleged member of the MS 13 gang, was detained near the U.S. Embassy and turned over to the FBI on Thursday.
The FBI said that Garciashot and killed 19-year-old Vanessa Argueta and her 2-year-old son with two other gang members in 2010.
One of the gang members was convicted of the murders by a jury and the other pleaded guilty.
Investigators said Garcia and Argueta, who had ties to rival gangs, dated but had a falling out before the shooting. Authorities believe the murder was an act of retaliation after rival groups threatened Garcia when the couple broke up.
According to the FBI, Garcia invitedArgueta to dinner on Feb. 4, 2010, taking her and her son to the woods instead.
With the other gang members, the FBI said Garcia executedArgueta with two shots from a handgun as her son watched.
"The gun was then turned on the child," according to an FBI release. "The first shot knocked him to the ground but did not kill him. The boy got up and clutched at Garcia’s leg, but another gang member shot again and killed him."
Garcia was added to the FBI's Most Wanted List on Wednesday for his role in the shooting.
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