Well fair enough, so here's my logic in a nutshell: IF it is a lone shooter, the odds are it is a domestic nutcase, just because there is a larger supply of domestic nutcases. But IF there is more than one person involved, it is more likely to be a terrorist cell, for two reasons: 1) The odds it is nutcases goes down geometrically for every incremental nutcase involved, i.e. if the odds for a certain demographic are one in 100,000, the odds two of them got together are one in 10,000,000,000 - one in ten billion, which makes it unlikely such a team of nutjobs even exists in the human population. 2) AQ trains for such actions - as in the Oregon cell - in small groups, and they also train multiple cells (as for 9/11) for the same op.
Even if you quibble with my "one in 100,000" number, it doesn't matter. Any fairly long odds - even one in 1000, which would make such killings a common event - get into the millions and then billions against once you add one or two co-conspirators. If it is a team effort, I'd give odds it is terrorists.