Bullshit. Terrorism sends its message by itself -- it doesn't depend on some later police/FBI investigation. If it did it would never work.
Terrorism isn't simply for the sake of terror. That's simple sadism. Terrorism's raison d'être is to convey a message. If a terrorist has no message to convey, then the act never happens -- because there's no point in doing it. And no, they're in no way "random" except as regards who the victims are. The act itself is planned, calculated and engineered
specifically to make a statement to the general public. Without the statement ... there is simply no point. And without a point, it's not terrorism.
The Badenov Brothers may have had a motive. But what they didn't have is a message. No message; no terrorism. Perhaps they might have
intended to plan an act of terrorism. If they did, they fucked up, because they failed to accomplish it. At most you have some abstract perceived revenge. You don't have terrorism.
Eric Rudolph didn't fail. Tim McVeigh didn't fail. Al Qaeda didn't fail. The messages were obvious and immediate, which is what they're supposed to be. You're not sending a message when you have to depend on a third party who's not even a participant to get the word out two weeks later.
As if a third party could be a reliable source to convey that message anyway.

Terrorism ALWAYS controls the message. When Ted Kaczynski put out his manifesto he didn't just jot down notes and tell the press "tell this in your own words" -- he demanded that it be printed verbatim, in
his words. Again, controlling the message is vital. It's the whole
point.
Terrorism is at base a political statement. You haven't made a statement by simply holding some belief and then not telling anybody what it is.
So your stretch here is absurd.