The linked December 2017 Russian media article indicates that Russia's GRU was behind the hack of the DNC in 2016 (and possibly also in 2015):
Title: "What the Arrest of the Russian Intel top Cyber-Crime Expert Has to Do With American elections"
https://thebell.io/en/arrest-russian-intel-top-cyber-crime-expert-american-elections/Extract: "Until now, U.S. authorities have appeared to base their conviction that the Russian government directed hacker attacks against Hillary Clinton’s presidential campaign on technical research alone. Reporting by The Bell reveals that Russian authorities believe four men betrayed their country by confirming the U.S. findings.
Sergei Mikhailov was arrested one year ago, on Dec. 5, 2016. Officers of the agency’s internal security division seized him at his office and led him away with a sack over his head. Mikhailov is a black belt in karate and the officers feared that he might resist, explained one of the colonel’s acquaintances.
Prior to his arrest, Mikhailov was head of the 2nd Directorate of the FSB’s Information Security Center (TsIB) and within Russian intelligence circles he was considered the main authority on cybercrime.
Now he and three other men — Dmitry Dokuchayev, an FSB major and former criminal hacker, accused in the U.S. of hacking 500 million Yahoo! accounts in 2014; Ruslan Stoyanov, a former Kaspersky Lab employee; and Georgy Fomchenkov, a little-known internet entrepreneur — are suspected of state treason. The four are being held in Moscow’s high-security Lefortovo Prison, in cells measuring nine square meters and without hot water, sources close to them say.
Classified as a “state secret”, the entire affair has been kept out of the public eye. “The four men have been hidden away from everyone, to make sure they don’t give away any sensitive information”, says Ivan Pavlov, a lawyer for one of the defendants.
The Bell has spoken to three people who are very familiar with the accused and their plight to discover why, in their opinion, the men are being prosecuted.
Two of the three people say the men passed the information about last year’s Russian attacks on the Democratic National Committee (DNC). Two sources also say that Russian military intelligence (GRU) was behind the arrest of Mikhailov and his companions. US Intelligence agencies are confident that GRU is behind the attacks on the DNC. The administration of Barack Obama imposed sanctions on the head of GRU Igor Korobov and three of his deputies, but not on any individuals in the competing Russian intelligence service, FSB.
One of the sources claims the defendants helped the Americans identify the hackers who broke into the DNC’s servers. The Bell was unable to confirm the claim. But the New York Times last January reported that the detention of Mikhailov and Stoyanov could be connected with hacker attacks on the DNC. It cited two U.S. officials as telling the paper that some Russian sources had played “a crucial role” in helping U.S. intelligence identify the culprits."