The publication has received a series of threatening messages on its Facebook page since June 8, and on June 22 a hand-written letter was posted to the newsroom containing the same threats and the words "Allahu akbar" (God is greatest).
"No one has been arrested at this stage and investigations are ongoing," said the source.
Jihadist brothers Said and Cherif Kouachi gunned down eight Charlie staff as well as several others in and around the building in the attack on the magazine whose drawings of the Prophet Mohammed drew the fury of Muslims around the world.
An accomplice, gunman Amedy Coulibaly killed a policewoman a day later and then took shoppers hostage at a Jewish supermarket, shooting dead four. All three jihadists were killed in police raids.
While the staff of Charlie Hebdo moved into new top-security premises, the magazine has continued to raise ire, refusing self-censorship in the wake of the attacks.