The Washington Post article, "National Security Russian government hackers penetrated DNC, stole opposition research on Trump" from June 14, 2016, states that the hacking group known as Fancy Bear "broke into the network in late April and targeted the opposition research...
Regarding the DNC's secrets being spilled in the last election, the question is, as the American Thinker's Mike Razar has put it, whether Wikileaks "received the information from the Russian government or from some non-Russian hacker source or even an inside leak...