http://www2.fireeye.com/rs/fireye/images/rpt-apt28.pdf
https://www2.fireeye.com/rs/848-DID-242/images/rpt-apt29-hammertoss.pdf
They are finding various correlations, such as Russian language settings, compile timestamps matching Russian work days, malware activity ceasing on Russian holidays...As a software developer, I can say that this "feels" par for the course for software developed in large organizations. Software that does it's job but obvious things, such as not realizing the developers workstation's locale names are being added to the binary, is forgotten.
Note also that the accusations against Russia hacking the DNC aren't coming out of the blue. The same evidence was there already in July when the emails were published by WikiLeaks. It wasn't until after the election that pundits started to believe something other than Russia was behind the hack.