BTW
https://www.dhs.gov/news/2016/10/07/joint-statement-department-homeland-security-and-office-director-nationalJoint Statement from the Department Of Homeland Security and Office of the Director of National Intelligence on Election Security
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 are intended to interfere with the US election process. Such activity is not new to Moscow—the Russians have used similar tactics and techniques across Europe and Eurasia, for example, to influence public opinion there. We believe, based on the scope and sensitivity of these efforts, that only Russia's senior-most officials could have authorized these activities.
In other words, NSA and ODNI directly state they're confident the email hacks etc were done by the Russians, and could only be done by senior state-level actors within Russia.
Some states have also recently seen scanning and probing of their election-related systems, which in most cases originated from servers operated by a Russian company. However, we are not now in a position to attribute this activity to the Russian Government.
And there was also direct evidence of Russian-owned servers trying to hack election machines in the USA, but they didn't have direct evidence linking that specific activity to the Russian government as of October when that statement was released.
So, no WaPo isn't the only source. This is a public document from the agencies involved.