Bombs go off near finish line of Boston Marathon

I find it odd that no-one has claimed responsibility for this.
Normally, they'd proudly declare "We did this! And unless you do as we demand, we will do it again!"

That's what concerns me the most, there seems to be no reason other than "Look, we can do it whenever we choose too" which is more scary than "OUR ORGANIZATION CONSIDERS YOU HEATHENS"