A marathon is the most unifying of sporting events. The city that shows up to cheer on thousands of runners doesn’t really know or care much about who wins; there are no sides to root for or against. Those who stand on the sidelines — as they have done in Boston since 1897 — come to celebrate runners from around the world. The country or neighborhood of origin of the competitors matters far less than their stamina.

On Monday, the weather for the 117th running of the Boston Marathon was cloudy and a little chilly — just the way runners like it. Three hours after the winners had broken the tape, there were still many runners on the course, and hundreds of spectators on the sidewalk, when an explosion rocked the finish-line area on Boylston Street, across from the main viewing stand. For a brief second, the flags of scores of nations were bent downward by the blast.

A few marathoners were knocked over by the force of the explosion. Some runners, locked in their trance, kept going until they realized something horrific had just happened. When they turned back, they said they heard the screams and wails, saw the column of rising smoke, and then the blood and limbs of victims. There was broken glass and agony everywhere.

Fifteen seconds after the first one, there was another explosion a few blocks away. It was clear this was not a random event but another concerted effort to kill and maim innocent Americans, just because they had gathered in a vulnerable spot on a day when no one’s mind was on terror. The police confirmed that bombs were responsible for the mayhem; three more unexploded devices were found elsewhere around the city. At least three people died — one of whom was 8 years old — and dozens more were injured, some severely.