Russia’s FSB security agency on Monday said the militants involved in the suicide bombing in St Petersburg in April, in which 15 people were killed, had used the Telegram messaging app to carry out the attack, AFP reported.

Russian businessman Pavel Durov is the founder of Telegram, an app that has about 100 million users. It is extensively used by the Islamic State group and its supporters.

FSB claimed that the suicide bomber had used Telegram to plan the strike with his accomplices as it provided them “with the opportunity to create secret chatrooms with a high degree of encryption”, Reuters reported. The agency believes that the accused had used Telegram “at each stage of the preparation of this terrorist attack”.

Russia has threatened to block the messaging app unless Durov signs the new data laws. Once the app is on the official government list of information distributors, it would have to store user information from the past six months on Russian servers. The company will also have to provide authorities with user data upon request.

However, the founder and CEO of Telegram has refused to co-operate with Russian security services, reported BBC. Telegram fears that if it signs the data laws, the privacy of more than six million Russian users would be at risk. “If you want to defeat terrorism by blocking stuff, you’ll have to block the Internet,” Durov said, according to Reuters.

On April 3, Akbarzhon Jalilov, identified as a Russian, had blown himself up in a train that was travelling between Sennaya Ploshchad metro station and Tekhnologichesky Institut station. The attack was claimed by a little-known group – Imam Shamil Battalion – that is suspected to have links with the Al-Qaeda.