No one knows for sure who hacked the French television network TV5 Monde on April 8, and that is part of the problem. The attack appears to have been well planned, and the hackers had probably infiltrated the network’s firewall-protected servers weeks before they struck at about 10 p.m. Paris time, hijacking 11 of the network’s television channels as well as TV5 Monde’s website, Facebook page and Twitter account simultaneously.

TV5 Monde’s director, Yves Bigot, said the network’s systems had been “severely damaged” in what he called an “unprecedented attack.”

The hackers posted black banners proclaiming “Je suIS IS” or “I am ISIS,” in a mocking reference to the solidarity slogan “Je suis Charlie” coined in response to the terrorist attacks in Paris in January. On TV5 Monde’s Facebook page, they ominously posted images of ID cards they claimed belonged to relatives of French soldiers combating ISIS, warning: “You have the chance to save your families, take advantage of it.”

Responsibility was claimed by a group calling itself the CyberCaliphate, the same name used by hackers who took over social media accounts of the United States Central Command in January and Newsweek’s Twitter feed in February. Hopefully, the investigation launched by the French government will reveal whether the attackers are part of the Islamic State’s hacking operation or another solo attack by a jihadist offshoot.