In October, the Department of Homeland Security and the Office of the Director of National Intelligence released a joint statement blaming the Russian government for hacking the DNC. In it, they state their attribution plainly:
The U.S. Intelligence Community (USIC) is confident that the Russian Government directed the recent compromises of e-mails from US persons and institutions, including from US political organizations. The recent disclosures of alleged hacked e-mails on sites like DCLeaks.com and WikiLeaks and by the Guccifer 2.0 online persona are consistent with the methods and motivations of Russian-directed efforts. These thefts and disclosures