The Boston Marathon is America’s iconic race, the oldest marathon in the country, and the most important. Eighteen people ran it in 1897; last year, thirty-five thousand did. It’s the site of Alberto Salazar and Dick Beardsley’s 1982 duel in the sun. It’s where Kip Litton may have ridden a bike, and where Rosie Ruiz definitely took the subway. The race is also iconic because you have to qualify. A New York Marathon shirt means someone got lucky in a race lottery. A Boston Marathon shirt means they’ve run fast. The finish line today was one of the saddest, most terrible athletic scenes ever. But in an ordinary year it’s extraordinary. Well-trained amateurs from all over the world: sweating, straining, slowing, sprinting.

The course was chosen to humble you. You start way off in Hopkinton, a town so far out on the Massachusetts Turnpike that it seems like it must be farmland. Then you run east. There are cheers at the beginning, and then it falls quiet for a spell. Families sit in lawn chairs clapping for the runners and listening to the Red Sox on the radio. At Wellesley College, at mile fourteen, the students come out en masse and cheer.

The course is slightly downhill for fifteen miles, then you blitz down what feels like a ski shoot. Suddenly you come to the bottom of the Newton hills. For the next four miles, just as your carbohydrates run out, you feel like you’re in the Alps. Four hills rise in front of you, but it’s hard to keep track when you’re spent. It can feel like five hills, or six. The course makes your mind falter right when your body does the same. And you’re still going east. Most marathons are run in some kind of a loop, which means that sometimes the wind is in your face and sometimes it’s at your back. In Boston, it’s all or none. In 2007, the race was run into gale-force winds. That year, the winner finished in 2:14. A few years later, with the wind blowing the other direction, the winner came in at 2:03. No one has ever run a faster marathon.

Soon, you’re stumbling past Fenway Park. The race comes to an end in beautiful downtown Boston. Copley Square. Newbury Street. Trinity Church. The swan boats—make way for ducklings. I’ve run the race twice and, one year, I was so exhausted after finishing that I spent forty-five minutes curled up in a corner in the Public Garden. I didn’t run this year. But I was in Boston yesterday, and I jogged just past that corner and saw a group of elite Kenyans gliding gently down the path.

Now all of that seems irrelevant: athletic frivolity at a time of horror. But it matters, too. We don’t know for sure who set off the explosions, and we certainly don’t know why. But iconic events are a temptation. Terrorists struck the World Trade Center, and they hit the Boston Marathon. The videos of people fleeing from the blasts, with the smoke curling up, are eerily similar to what we saw twelve years ago—and what we’ve seen other times too, as in London and Madrid.

There’s something particularly devastating about an attack on a marathon. It’s an epic event in which men and women appear almost superhuman. The winning men run for hours at a pace even normal fit people can only hold in a sprint. But it’s also so ordinary. It’s not held in a stadium or on a track. It’s held in the same streets everyone drives on and walks down. An attack on a marathon is, in some ways, more devastating than an attack on a stadium; you’re hitting something special but also something very quotidian.

When we find out who did this, we may well find some fascination with the event—perhaps a foreign terrorist, or a sick American. Perhaps it was someone who spotted a terribly easy target. Or perhaps it was someone who saw a reflection of the human spirit and decided just to try to shatter it.

Read more of our coverage of the Boston Marathon explosions.

Photograph, of a woman near Kenmore Square after two bombs exploded during the Boston Marathon, by Alex Trautwig/Getty.