i suspect this is all about buying more time for the world to get over the shock of this terrorist attack, thereby hopefully preventing for a few more days the end of the world.
as for 'valid reasons' -- i can't think of any.
as for likely suspects, only one country that could do it and expect to get away with it without any serious repercussions -- the US.
and, by extension, our protected partners in aggression -- the UK, in particular. also Nato.
Israel, i suspect, would be out because of close ties to Russia, but you never know.
Russia has officially said the UK was involved in the attack. the UK has offered the Shaggy defense.
the US had the most to gain, and Russia had the most to lose.
so the official word, so far, from most western media including every mainstream pr firm has been:
Russia did it! Self-sabotage! They're so dastardly with their false flags!
our intellectual culture is in bad shape.