Federal agents say an ISIS cell communicating over Telegram planned to bomb St. Petersburg's Kazan Cathedral

Russia’s Federal Security Service (FSB) says it’s detained seven ISIS members in St. Petersburg who were planning to stage an attack on December 16. One of the suspects later announced that his group had planned to detonate a bomb inside the Kazan Cathedral, one of the city's most recognizable pieces of architecture. Federal agents say the terrorist cell communicated with ISIS leaders using the instant messaging service Telegram.

In April 2017, a suicide bomber attacked a subway train in St. Petersburg, killing 15 people and injuring roughly 100. Ten individuals have been detained in the investigation into the bombing. The two men suspected of organizing the attack claim they were arrested illegally and tortured at a secret prison.

The FSB has ordered Telegram to surrender the encryption keys for all user correspondence, arguing that terrorists use the service. A Russian court fined Telegram 800,000 rubles ($13,580) for ignoring the FSB’s orders, but the tech company is trying to challenge this decision.