Boston Tragedy

My heart goes out to those affected by the Boston bombing. I have family that live around Boston but  thankfully they weren't around the ill fated festivities that day.
I still however fail to understand how they can't catch things like this before they actually happen considering their heightened state of security in that country. One man from Alberta even mentioned he saw sniffer dogs near the start line of the race. That's the part that I found most interesting and I have highlighted it for your viewing pleasure. Why did the sniffer dogs at least not notice one of the four bombs located at various points of the race?
What a tragedy and I hope for the fastest recovery to those victims affected.