Let me make this easy for everyone: all spree killing and mass murder is a form of terrorism.
Whether or not there is any stated manifesto, the psychological goal of producing a spectacle to show the public the mighty wrath of the aggrieved is the same. All mass killers want to draw attention to themselves and their issues. All mass killing is a political act in that the public spectacle is an attempt to get the body politic to hear, feel and react to the aggrieved. Some such terrorists are more or less mentally competent than others, and more or less coherentabout their grievances, but the techniques of terrorizing the public to air their grievances are the same.
"Professional" terrorists differ from "freelancers" in that they have an organizational affiliation, just like the difference between syndicate hitmen and private multiple murderers. The professionals are still violent sociopaths, they just trend towards the more stable and coherent side of that spectrum. They are still able to kill because of something in their own personal psychology, just like the independents. And some of each type plan to terrorize multiple times, while others in both camps (suicide bombers, those seeking "death by cop") plan to go out in a blaze of glory. They're all still terrorists.
So let's get away from this bigoted and analytically incorrect "Muslim Terrorist" versus "White Mentally Troubled Lone Wolf Murderer" dichotomy and accept that all mass murderers are terrorists. Ones with organizational affiliations (the 9/11 terrorists, McVeigh & Nichols) are "Organized Terrorists", and those who are just going it alone (Kaczynski, the Dzhokhar brothers) are merely "Terrorists".
This will help reduce bias and make the law enforcement analysis and response to such actions more effective. They should always look for organizational ties, and always be willing to accept that maybe there are none, and in both cases call the action what it is: terrorism.