That's a very good question.
It's an absolutely terrible, terrifying episode, and I'm speaking as one who has spent quite a bit of time in St. Petersburg. I know the names of the subway stations well. Sadly, it's not the first such awful attack in Russia. The subway in Moscow has had several bombings in the last decade. I think the last one was in 2010. There was an earlier one in 2004. Then, in terms of terrorism in general, there were some explosions just on the eve of the Sochi Olympics when a bus station in Volgograd was blown up.