Vladimir Putin propped up Bashar Assad instead of fighting Islamic State.
https://www.wsj.com/articles/terror-returns-to-russia-1491347724
https://www.ft.com/content/9a10c3d4-1951-11e7-9c35-0dd2cb31823a
As Russia gets more involved with the Syrian civil war, it's likely that Sunni militants will intensify their campaign against Russia. But the key reason why Sunni attacks on Russia proper will increase is the fallout between Sunni jihadists in the Caucasus — namely, the al-Qaeda-linked Caucasus Emirate and the ISIS-linked Wilayat Qawqaz.
http://www.rand.org/blog/2017/04/attacks-on-russia-will-only-increase.html
http://ru.krymr.com/a/28410136.html
http://carnegie.ru/commentary/?fa=68519
http://carnegie.ru/commentary/?fa=68526
https://www.inopressa.ru/article/04Apr2017/independent/terror1.html
https://www.novayagazeta.ru/articles/2017/04/04/72020-oni-prosto-byli-zanyaty
https://politeka.net/ua/423160-vybukhy-v-sankt-peterburzi-komu-vyhidno/