Seems risky to think you'd be able to get away with this without someone developing some reasonable suspicions. Which makes me lean towards one of the Western parties. Don't think Russia would be able to pull this off without getting caught and if caught you'd think the US would be trumpeting the proof from the roof tops.

It would be a high risk play but Russia is likely the one to lose the least from it if caught given their current position. Russia is also a nuclear power so even if the US had proof they may not want to escalate the conflict that much. There's also the question of the truth even mattering since in the age of constant online propaganda you can influence enough people to have an impact even if the documented truth is otherwise. Then again the current US administration may feel the risk is worth it for a better chance in the upcoming election.

This is a classic FSB/KGB propaganda technique ("So you're saying there's a chance" --Kruschev). This is 99.9% the russians.