The Russian messaging app Telegram, reportedly widely popular with ISIS, was restored after going out across Europe and the Middle East for almost two hours on Wednesday.
The messaging app, which claims to have 100 million active monthly users, had intermittent issues over that time with some lingering outages in Germany, according to the news website RT.
"Some of our users in Europe are experiencing connection issues at the moment. We're hard at work fixing this," the Telegram had said in a Twitter post, asking for patience among users of the encrypted and somewhat controversial messaging service.
Telegram notified users Wednesday on Twitter that the service was back up.
Telegram was started in 2013 by brothers Nikolai and Pavel Durov. It provides a messaging service with optional end-to-end encryption, RT said. The brothers also created the Russian social networking site VK.
The Huffington Post reported in May that Telegram had become the top way for the Islamic State militants to communicate, distribute information and bring followers together. The app is preferred for providing better privacy as Twitter and Facebook have cracked down on terrorists groups.
"We've 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.
ISIS's move to Telegram appears to be part of the more expansive effort toward private networks and the "dark web." Experts say that terrorist groups have become more proficient at altering their communications strategies on the Internet, HuffPost noted.