There is extensive research documenting Russian interference in the 2016 US presidential elections - see under US Elections 2016 here.
Government investigators independently verified that Russian operatives hacked the Democratic National Committee in 2016 and did not rely on the findings of a private cyber firm, said the US prosecutor in a court filing.
The prosecutors were rebutting a claim made in a prior court filing by Stone — a long-time Trump adviser who is fighting charges of making false statements to Congress, obstruction and witness tampering — that the government relied only on “an inconclusive and unsubstantiated report” written by cyber research firm CrowdStrike and did not “collect any evidence of the DNC breach directly.”
While the prosecutors noted that the investigators gathered evidence of the Russians’ involvement independently, which led to the indictment of 12 Russian military officials (in connection with the DNC hack). In the detailed indictment, one can read: "Lukashev used the account to mask a link contained in the spear phishing email, which directed the recipient to a GRU-created website (...) On or about March 21, 2016, Lukashev, Ermakov, and their co-conspirators stole the contents of the chairman’s email account, which consisted of over 50,000 emails" and other details.
See also the article in the New York Times, detailing how the FBI discovered the hacking of the Democratic National Committee’s (DNC) server 7 months before the Democrats hired CrowdStrike to investigate.
See more disinformation cases on Mueller's report, also known as Russia probe, or Russian meddling in the 2016 US and elsewhere elections.