Of course all we can do is speculate and make educated guesses at this stage, but having read many of the previous postings on this subject, I am suprised that the "T" word (terrorism) hasn't been mentioned more often as a theory today?
As much as we would like to brush this aside, I am sure that "radical groups", especially AQ etc are not considered a threat in south america, and perhaps the terrorists have realised this too and have managed to breach security in Rio?
Let's hope not!