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.
The bombers are just as gutless and cowardly as every other organisation that targets innocent civilians in its attempt to achieve its own warped agendas or exact some form of twisted revenge.
The car bombers in Iraq are no different. With a week to go before elections there, terrorists have stepped up their campaigns, setting off car bombs in markets and shopping centres - areas where civilians gather to shop, socialise and try to pretend that there is some semblance of normalcy in their lives.
The death toll is rising by the day and, as always, the innocents are suffering. If the targets were government or military institutions, it would be easier to understand. There is a clear enemy.
But I have never understood the rationale of targeting civilians.
When I visited Ireland in 2000, a book had just been published listing the name and biographical details of every person who had been killed in the Troubles. A kind of macabre Who's Who.
There was the young bride-to-be and the girls who were to be her bridesmaids - killed and horribly wounded in their local pub as they gathered over a pint to discuss dress fabric. The 15-year-old schoolboy whose last words were, "Tell my Mummy I love her." More than 3000 people who were so much more than collateral damage.
The only thing we can do is refuse to give in to the terrorists. We must refuse to be terrorised, and keep living our lives. Keep trusting one another. Keep believing that, ultimately, goodness and decency will prevail.
Because it does. And it will.