A cybersecurity expert tells CNN that his team did catch Russian hackers in the act of hacking the DNC back in May of this year:

In an interview with CNN’s Wolf Blitzer, Dmitri Alperovitch, whose firm investigated the DNC hack, said that their firm did, in fact, catch two groups associated with Russian intelligence agencies in the act.

He wouldn’t say who the groups were, only that they were associated with the Russian government.

More here:
http://therightscoop.com/cnn-expert-says-caught-russian-hackers-act/