It was what marathon organisers in America had been fearing since 9/11.
Marathons, with their tens of thousands of participants and their hundreds of thousands of spectators and their 42km of course, are the ultimate in soft targets.
It is impossible to protect so many people over that amount of territory no matter how rigorous and comprehensive security procedures may be.
There has to be a certain level of co-operation and trust for an event of this scale to succeed and that's precisely what the evil malcontents who let off bombs designed to maximise hurt and harm depend upon.
They prey on people's goodwill and don't care who their casualties may be. Women, children, the elderly - it's all the same to them.