It was derived from the two, possibly three suspects that could have launched the attack.
A poor methodology. Two suspects who might have done it does not make it a 50/50 chance and attempting to derive a likelihood from that alone is embarrassingly poor reasoning. Especially when more information actually is available.
No other information is available so that's about the best you can do.
I didn't post those links for my health, you know. :-|
If you know something no one else knows, then that would change the 50/50 odds, but since you don't, you have no more to go on than faith, something common in theist arguments by the way.

Oh, so close!
This is the same argument I had with holders of faith in Saddam's WMD 10 years ago.
Always with the Iraq, as if they were identical situations. They're not, and neither the situation, the reasoning behind intervention, nor the scale of the intervention is similar. In one case, a brutal civil war and humanitarian-disaster-in-progress that has recently gone WMD and threatens to deteriorate even further. On the other, a brutally oppressive but largely contained situation where unilateral action was pre-planned and the public and international community were deliberately misled.
Iraq was so terrible and so profoundly jarring that I'm not surprised that people are still stuck in Iraq mode, reacting to a new situation as if it were the old one. It's fortunate that not everyone knee-jerked that way.
As far as you can tell. However, that is clearly not the case. And these predictable accusations (even if they weren't horribly, horribly mistaken) do nothing for your argument, such as it is.