I saw a poll once that well over 80% of federal government employees consistently votes Democratic compared to a close to 50/50 split for the rest of the country. In doing some moderate research to find that poll, I stumbled across this link that shows the amounts of money contributed by employees of various federal government departments to the two parties:
http://www.fedsmith.com/2016/05/19/whic ... employees/You will note that employees of the Department of Justice, the department that the FBI is under, during the 2015-16 campaign season contributed $137,603 to the Democrats while donating less than $15,000 to the Republicans. That's 90% Democrat, 10% Republican, which corresponds pretty closely to the polls I've seen.
With that kind of bias, I don't trust the FBI to investigate matters of a political nature, so it wouldn't surprise me a bit if the FBI was covering up for Hillary.