It would seem odd that your average extremist ultra-nationalist would use a professional silencer.  This comes from RFE/RL:

Svetlana Gannushkina of Russia’s Memorial human rights center told RFE/RL’s Russian Service that Markelov’s murder bore the signs of a hired killing.

“This could not have been accidental, and there was certainly no criminal motive behind it,” Gannushkina said. “Unfortunately, we cannot conduct our own investigation. We can only demand that the law enforcement bodies do it. But our capabilities, our mechanisms, so to speak, of making such demands are unfortunately very, very weak. All we can do is express our indignation and try to find out who threatened Stanislav, when, why, and how.” (…)

In an interview with RFE/RL’s North Caucasus Service, Kungayev — who now lives in Norway with this four remaining children — said he believed Budanov was behind the slaying.