You're a bit mixed up on this.  The intel community definitely did forensics work on this and were able to analyze the malware put on the DNC networks.  They discovered the Malware X-Agent and the malware X-Tunnel.  X-Agent opened up a backdoor that allowed the hackers into the network to view, monitor, copy files etc.   7ff3   When they gathered what they needed, they used X-agent to exfiltrate the files onto a different server.  The Intel community was able to track the use of this malware to two separate leased computers in the US and then track back the proxies until they were able to identify which GRU officers were controlling the systems.    Here's what Admiral Rogers said about this  Adm. Mike Rogers, director of the National Security Agency, is directly blaming Russia for the data breaches at the Democratic National Committee (DNC), which he called "a conscious effort by a nation-state to attempt to achieve a specific effect." At Tuesday's Wall Street Journal CEO Summit, Rogers brushed off claims by WikiLeaks head Julian Assange and others that Russian President Vladimir Putin's regime was not behind the attacks, as well as remarks from President-elect Donald Trump that anyone could be responsible. “There shouldn't be any doubt in anybody's mind. This was not something that was done casually. This was not something done by chance. This was not a target that was selected purely arbitrarily," said Rogers at the event. "This was a conscious effort by a nation-state to attempt to achieve a specific effect."  This was published in The Hill dot com under the headline  NSA head: Dem hacks were 'a conscious effort by a nation-state'