The attribution of the DNC hack to a Russian APT group has some backing from security professionals. Crowd Strike, based on analysis of the malware used, came to this conclusion (https://www.crowdstrike.com/blog/bears-midst-intrusion-democ...). This conclusion has been corroborated by Fidelis (http://www.threatgeek.com/2016/06/dnc_update.html) and Threat Connect (https://www.threatconnect.com/tapping-into-democratic-nation...). It's early days, but the evidence is there.

FYI, the Sony hack was very likely not committed by an insider (https://www.operationblockbuster.com/wp-content/uploads/2016...). DISCLAIMER: I worked with the team who did operation blockbuster.