At about 2:45 PM EST, two explosions detonated within yards of the finish line of the Boston Marathon. The news is not official, but from the nature of the explosions and the coincidence of the proximity to the Marathon, it would seem probable that these were bombs. Preliminary reports indicate at least two other bombs were found and deactivated. So far, officials report two dead, 28 injured, and we can assume that count will rise through the night.
US stocks were having an off day anyway, so today's decline of about 2.3% in the major averages really has nothing to do with the explosion. Sad but true - these events are always a human tragedy, but never an economic tragedy. On July 7th, 2005, 4 suicide bombs in the London transport system took the lives of 52 Londoners, with no impact on stock prices beyond the initial shock. We will NOT be buying or selling stocks on the basis of this news.
What is of import is that if this event was indeed a terrorist attack, it would be the first successful attack on US soil since 9/11/01. Since 9/11, there have been 29 failed terrorist attacks in the United States.