I was part of the huge crowd watching the Boston Marathon, and cheering on wheelchair competitors, runners and the community of runners, and assorted other people celebrating health and exercise. The elite runners were remarkable, of course, but I found the folks running hard to achieve a time somewhere between 4 and 5 hours to be at least as worthy of cheering. It was a very uplifting experience, right up until the time we heard of the bombings.
I have no idea what the total security resources available to Boston and its surrounding communities, and the Commonwealth of Massachusetts, is but the presence of State and local police, emergency personnel, National Guard, etc. had to be very close to a maximal effort. There were police lining the course, on both sides. We were near the half-way point, and officers were about 25' apart, plus police roaming in automobiles and on motorcycles. It was amazing.
Of course, there were people everywhere, all carrying backpacks and coolers. Neighbors were engaging neighbors, and children were playing and slapping hands with the runners. There were probably spectators 4 or 5 deep, elbow to elbow, for 26 miles. I haven't tried to do the math, but that might be a million people. It was simultaneously a celebration of Spring, a huge party with all of the foolishness that entails, and an inspirational event featuring all those athletes, most of whom were people just like the rest of us who nevertheless managed to get off the couch and move, long and hard, and in so doing achieve something marvelous.
I do not know whether something else could have been done, but I think bombings like these should be hammering home the notion that we must meet these things with courage, deeply justified anger, and a determined police response, but we can't adopt security measures so extraordinary that we will always be safe from this sort of evil.