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They seem to believe that the bombs were detonated line of site, by radio remote control.
I'm not sure why they conclude this next part: Ergo, the Tsarnaev's themselves could not have detonated them, and so there is a third party who manned the RCs.
I do not get that last leap, though, as I thought that last picture of the murderous brothers leaving the bomb site had them pretty close to the bombs. Not so close to be hit, but close enough to be moving with the fleeing crowd (and not even out in front of the fleeing crowd).
But it's worth keeping an eye on. I await the explanation for that Step 2 in the logic that seems to be missing at the moment.