There's a lot of evidence in the public domain as to why the intelligence community concluded that Russia was behind the DNC and John Podesta hacks. Some of it was made public before the election: CrowdStrike recognized the specific software tools from attacks on other Russian targets; evidence of a Cyrillic keyboard used in the metadata, the suspiciously not-Romanian behavior of "Guccifer 2.0, malware which doesn't "call home" on Russian holidays, and bitly addresses were created from the same account as attacks on other Russian targets.

After the election, the newspaper The Hill summarized: "Five Reasons the Intel Community Believes Russia Interfered in the Election."

Enough detail is known now that prosecutors are considering bringing charges against specific Russians. Other targets of the attacks were linked only by Russian antipathy to them.