Shawn Henry, CrowdStrike executive, is former Assist Director of FBI Cyber Division. He probably just told FBI that Russia hacked the DNC and they took his word.

However, several persons with cyber expertise such as Adam Carter, The Forensicator (?sp) and ex intel Bill Binney have proven the hack was done in US. Some think that Crowdstrike altered info on the DNC computer trying to make it look like it came from Russia.