The federal grand jury in Virginia charged Gerardo Figueroa Sepulveda, 38; Omar Fabian Valdes Gualtero, 27; Edgar Javier Bello Murillo, 26; Hector Leonardo Lopez, 23; Julio Estiven Gracia Ramierez, 30; and Andres Alvaro Oviedo-Garcia, 21, with two counts of second-degree murder, one count of kidnapping and one count of conspiracy to kidnap, the DEA said in a release.
Oviedo-Garcia was also charged with two counts of assault.
The grand jury indicted a seventh Colombian national, Wilson Daniel Peralta-Bocachica, 30, for allegedly trying to destroy evidence associated with Watson's death.
Watson was abducted and killed June 20 while working for the U.S. Mission in Bogota, after getting into a taxicab operated by one of the defendants.
Watson's assailants used a stun gun on him and then stabbed him to death, the indictment alleges.
"Special Agent Watson was a brave public servant who dedicated his life to protecting the country he loved," U.S. Attorney General Eric Holder said in a statement.
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