New conversation screencap between Guccifer2.0 and actress @robbin_young indicates that Seth Rich leaked the DNC emails.
Seth Rich was a DNC Staffer who was murdered in Washington D.C. in July 2016. He was shot at 4:20am and news reported that it was an attempted robbery, though nothing had apparently been taken from him.
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woman: True love, can NEVER be bought!
woman: Who is this person? (25 Aug 2016)
guccifer2.0: his name is seth, he was my whistleblower
guccifer2.0: i suppose u know who i'm talking about
woman: Yes, and it was horrifying to hear about his death. I have no facts, but it's my opinion Hillary had him murdered. (25 Aug 2016)
woman: OMG, you MUST be VERY careful!
woman: What is the issue with your assistant? Do you feel you can't trust him or her? (25 Aug 2016)
guccifer2.0: it seems i can trust him, but he can't be a public figure asking questions. this will endanger him