Here's one relating to the most recent shooting. The other incidents I mentioned were widely covered in the news.

http://q13fox.com/2017/01/24/shooter-sent-facebook-message-t...

Here's an OK summary of trends in extremism within the US, including both Islamic and far-right varieties. The SPLC documents such issues pretty comprehensively and is a good place to start learning about the subject.

https://www.splcenter.org/20150212/lone-wolf-report

Oh and here's something about the wonderful folks over at Stormfront:

https://www.splcenter.org/20140401/white-homicide-worldwide