The facts reported so far, IMHO don't add up to a "lone suicide". To believe that story you have to ignore a _mountain_ of evidence to the contrary (evidence reported by many more people than just "questionable" websites and through numerous "official but anonymous" sources).
I certainly hope the FBI won't let any jihadis go free, but if they aren't even looking in that direction how can we be sure they won't?
I agree that the FBI might not show it's hand in this if there is an ongoing investigation (meaning a wider plot), but why wouldn't they release the file if it would prove it was a lone suicide?
I'll personally stick to asking questions, and analyzing what's been reported so far, and remain skeptic regarding anyone who knows "FOR SURE" one way or another until all the evidence is presented... if that day ever comes. And yes, even then I may still have my doubts.
It is an ongoing investigation, but that doesn't mean it's a wider plot. At any rate, I'm ~95% confident the kid's a schitz, regardless. The other folks in his building are lucky he didn't blow the place when he was whipping up his crap.