Any particular reason you don't think it was Russia?
Skepticism of course stems from decades of government lies and misrepresentations of truth in order to justify horrendous actions ie. what was done to Iraq, Lybia, Syria.
(Note I'm talking about espionage in the information gathering sense, not the overthrowing of regimes action movie sense)
I really question this assessment. Russia is not USSR, not by a long shot. Russia has lost an entire generation of the most educated people it did have, and I see no evidence that it ever recovered or will ever recover. Economically tiny and outright insignificant Russia simply isn't capable of developing serious technological capabilities to rival that of the US.
It's not that the USA couldn't match them, it's just that Russia has made it more of a priority. Tactically speaking, it's an excellent force multiplier with (varying degrees of) plausible deniability.