Prosecutor Marcelo Pecci, 45, known for fighting organized crime, was shot dead on the island of Baru near the Caribbean city of Cartagena while honeymooning with his wife.
Four people have already been jailed in Colombia for their involvement in his killing, and a man in Venezeula was arrested in December.
In the latest arrests, brothers Andres Perez and Ramon Perez are accused of arranging the logistics behind the murder, including hiring hitmen, who they planned to pay large sums of money, the prosecutor's office said in a statement.
Attorneys for the Perez brothers could not be reached for comment.
"These two people, it appears, were the ones who were in charge of the logistics, which made the criminal act possible," Colombian Attorney General Francisco Barbosa told reporters.
Officials seized documents, clothing and a car that had been used during the planning of the murder, the prosecutor's office added in a statement.
(Reporting by Julia Cobb, Writing by Daina Beth Solomon; editing by Diane Craft)
