You can’t use the English language in an openly misleading fashion and expect that people are going to go along with it, not in a situation like this. The expression “someone set this up” clearly implies the existence of a second person.
What made you think that another person is involved? Nothing. If we were to look at historical evidence, we would find that a lot of these situations are done by so-called lone wolf attackers. So if we’re going to blindly speculate, we should at least be consistent with historical evidence, and we should certainly speak unambiguously.