The ideal would be this horror (like so many others in the world) would never have happened in the first place. All we have is conjecture, of course, but if I had to choose between evils, the lesser of three evils, my hopes would be like this:
Least of Evils (Foreign Attack):
This is by far my hope because this is the kind of thing we can wrap our minds around. It is horrible but it is far better to believe this than think Americans murdered and maimed Americans, civilians no less.
Lesser Evil (Lone Wolf Lunatic):
If it was an American, awful as that would be, my next hope would be that it was one isolated nutjob. This too we can wrap our mind around. He/she is crazy and we can attribute this kind of evil to mental defect or the person being an extremist but a statistical abberation.
Homegrown Evil (Domestic Group):
This is the thing I fear the most, that there is a group of people behind this attack and they are homegrown extremists. We can't lay it off on outsiders. We can't lay it off on brain tumors or statistical abberation. This would be a group of people who talked, planned, and made the cold-blooded choice to murder and maim innocent people to send a message or just sow chaos.