Understanding here lies in understanding the difference.  Thus far there is no evidence that russians hacked into, lets say computers that count ballots.  There is evidence that they attempted to influence the election by hacking the DNC/Clinton campaign and allegedly giving that data to wikileaks to release in a drip, drip. 

The FBI deals with domestic intelligence and *law enforcement*; the CIA is deals in foreign affairs (espionage) and operations; they are not about law enforcement - the FBI has to have evidence they can take into court.