“How do we know that manifesto was written by the guy pictured below it?”
You don’t!
The police state agencies don’t disclose their operatives and informant, so why would engineering a manifesto be band consideration? They undoubtably are following a lot of disturbed dudes and when something like this happens, how do we know if it’s not made up and planted crap? Look, they helped to clean up hitlery’s files, so why not make up some files? The police state agencies want your guns. We know this from fast and furious and all the other crap they do. The FBI has a list of undesirable people and undesirable views (biblical Christianity, 2A, BOR etc).
That little garbage exonerating Trump from any culpability is the type of devious crap these goons would dream up to add “credibility”.
https://www.theguardian.com/world/2011/nov/16/fbi-entrapment-fake-terror-plots
https://www.usatoday.com/story/news/nation/2013/08/04/fbi-informant-crimes-report/2613305/
http://www.freerepublic.com/focus/news/3768947/posts?page=253#253
http://www.freerepublic.com/focus/news/3768947/posts?page=252#252