The press freedom watchdog group also says it has become a civil party to the investigation.
De Bourgues was found dead on August 2 in the apartment he was renting in Almaty, the former capital of Kazakhstan.
De Bourgues was in Almaty to prepare a report on Kazakhstan for SML Strategic Media, a U.S.-based media-consultancy group.
Kazakh authorities say burglars are responsible for the killing. But Kazakh media watchdogs suspect foul play.
RSF representatives traveled to Kazakhstan in September to investigate De Bourgues' death.