[Cboldt]

-- When did the FBI start monitoring the DNC servers ? and they would have needed a warrant ? --

Different sources ...

September 2015: FBI Informs DNC of Russian Hacking - The Moscow Project - NYT

The Perfect Weapon: How Russian Cyberpower Invaded the U.S. - NYT

Looks like FBI awareness may have arisen from reports to it, from others. Phishing email, perhaps with "To" headers that included DNC, or maybe the intrusion was used to send further phishing spam, this originating from DNC servers. You've likely received e-mail from a friend whose system was infected.

Yared Tamene, the tech-support contractor at the D.N.C. who fielded the call ... "I had no way of differentiating the call I just received from a prank call," Mr. Tamene wrote in an internal memo, obtained by The New York Times, that detailed his contact with the F.B.I.

[stylin19a]

Thanks. - you are probably right but it's still not that clear...the Feds were chasin something earlier...from the same Times article:

But in 2014 and 2015, a Russian hacking group began systematically targeting the State Department, the White House and the Joint Chiefs of Staff. “Each time, they eventually met with some form of success,” Michael Sulmeyer, a former cyberexpert for the secretary of defense, and Ben Buchanan, now both of the Harvard Cyber Security Project, wrote recently in a soon-to-be published paper for the Carnegie Endowment.
Harvard Cyber Security Project is a lefty place .

Why wouldn't the FBI be involved in that chase, especially on the WH ?

Interesting read here to include footnotes.
http://carnegieendowment.org/2016/12/13/russia-and-cyber-operations-challenges-and-opportunities-for-next-u.s.-administration-pub-66433

Even more so, just don't know what source to rely on anymore.