McVeigh did have accomplices and a lengthy history of opposition to government that played out increasingly after the Waco siege.  He also had a political agenda, albeit an anarchist one, but to your point, no known organization.  

While we have more to learn about the shooter, to me the actions taken speak more to a crime of hatred, a la Marc Lepine, than a crime of terror.  The way I see it, one isolated person's frustrations and violent actions do not describe or support a cause worthy of being linked to terrorism.  The problem is with the person, and the target chosen is incidental to that person's problems. 

That being said, my heart goes out to members of the Islamic community who may feel threatened by this deranged person's actions.  It's not you, it's him.