and something to add to what i said before, and to answer the original question, that is, how is USA responsible for it?
USA is responsible for it as USA projects itself to be responsible of taking down everything happening in every corner of world which goes against her “plans” and dream. USA is responsible because it happened on their land, just like USA said Afghanistan is responsible because osama was hiding there, even he was not even afghan. USA is responsible because this dreaded act of hate was only possible because of their faulty “freedom of speech” mess and their dual face towards the muslim world. yes USA is responsible for not doing enough.