I have no idea
To be perfectly honest, it wouldn't shock me if it was the FSB themselves.
Whether there are people high enough in the FSB and are also part of this NRA, no clue but it's possible
Why do I think it's the FSB? Well for one, the car was in a very secure car park prior to the bomb going off, it was expected that the father would have been driving so that again needed knowledge of expected movements and the FSB are known to have elements, very much at odds with some of the ideologues that have Putin's ear.
The only way it is them, is if they have supporters high up in the security services but I also wouldn't rule that out completely
One thing it isn't and thats a Ukrainian Secret Services female Operative that escaped to Estonia (as announced by the FSB)