Ghislaine Dupont, 57, and Claude Verlon, 55, were kidnapped and shot dead by what French Foreign Minister Laurent Fabius said were "terrorist groups" in the flashpoint northeastern town of Kidal on Saturday.
"A few dozen people have been arrested on Malian territory over 48 hours in the course of the investigation related to the murder of the two French journalists," a source from the Kidal administration told AFP.
A member of the Malian security forces confirmed the information, putting the number of people detained at "at least 35".
The source told AFP evidence was "accumulating" and amounted to more than just "clues".
He said prisoners serving sentences for their involvement in the kidnapping of Frenchmen Philippe Verdon and Serge Lazarevic in northern Mali in 2011 had been helping the investigation.
Verdon, who was subsequently murdered, and Lazarevic, who remains a hostage, were kidnapped in the town of Hombori by a group contracted by Al-Qaeda in the Islamic Maghreb (AQIM), some of who were then arrested.