In the hours after the deadly terror attack on Monday in Manchester there was a flurry of activity on the Islamic State militant groups favorite app, an encrypted messaging service called Telegram. Even before authorities released details on the attack, ISIS supporters flooded the apps private and public channels with celebratory messages.

Telegram has become one of ISISs primary means for disseminating information and bringing together supporters. Following Mondays bombing, it was a hub for pro-ISIS propaganda and ultimately a source for the groups official claim of responsibility.

As social media platforms such as Twitter and Facebook increased their efforts in recent years to shut down pro-ISIS accounts, Telegram has filled the void. Since around 2015, analysts say there has been an exodus of extremists to the app in search of better privacy and the freedom from being shut down by moderators.

Weve seen a clear trend of growing use of Telegram by almost all terror groups all over the world, Gabriel Weimann, a professor at the University of Haifa in Israel and author on online extremism, told HuffPost.