For the city, today’s tragic explosions at the Boston Marathon finish line could not have occurred at a worse time or place. Owing to the rich history and prominence of the annual race, there is no other time of the year when the city is more crowded with locals and visitors alike. Runners and spectators from across the country and around the world converge on the city every year for the celebrated Patriots Day race. And the explosions erupted at the race’s end when hundreds of runners were completing their 26-mile feat to the cheers of well-wishers.

Whether the explosion was the work of an organized terrorist group or the action of amateurs seeking to create chaos and confusion, this calamity differs significantly from such crimes as the Oklahoma City bombing and the 9/11 attack. Earlier terrorist strikes were perpetrated when least expected — a routine day for all those caught by surprise.