Jihadist propaganda messages were posted onto TV5Monde social media accounts in April.
Russian hackers are the prime suspects in the cyber attack that took place on French television network TV5Monde, in April this year.
Hackers managed to disrupt transmission on the TV5Monde’s network, and posted pro-ISIS propaganda messages on the network’s Facebook and Twitter accounts.
L’Express newspaper cited a judicial source as saying: "The investigations are at this stage looking towards a group of Russian hackers designated by the name APT28."
The APT28 has been previously accused of hacking the networks of White House and NATO members, and had targeted Ukrainian activists.
The investigators reportedly figured out the culprits of the attack by probing the IP addresses of computers used in the attack, reported AFP.
As a result of the attack, the network had to broadcast pre-recorded programmes, as it could not resume operations for 18 hours after the hack. Later, TV5Monde CEO Yves Bigot said that the cyber-attack "unprecedented in the history of television."
Some of the posts on TV5Monde’s Facebook after the attack read: "Soldiers of France, stay away from the Islamic State! You have the chance to save your families, take advantage of it.
"The CyberCaliphate continues its cyberjihad against the enemies of Islamic State."