Juan Carlos Martinez was killed while driving to his home in a suburb of the Guatemalan capital, La Prensa Grafica reported Tuesday.
Martinez, killed Monday, was investigating the deaths of Eduardo D'Aubuisson, William Pichinte and Jose Ramon Gonzalez, who were members of the Central American Parliament, were found dead in their car in February 2007.
Guatemalan police said they suspected drug gangs were behind the killings.
D'Aubuisson was the son of the late founder of El Salvador's ruling Arena Party, Roberto D'Aubuisson.
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