It happened when we had almost stopped thinking it could.
We had come to believe we were safe, that terrorist attacks of this sort were a relic of our unprepared past, something that wouldn't happen again.
And then it did. Explosions. Smoke rising. Sirens screaming. Ambulances racing. State Police SWAT vehicles lumbering around the streets. National Guard troops on the Common.
Raymond Kwan, 60, a chemical engineer from Houston, had finished the marathon just minutes before and was a block away when the explosions came.
"I finished a little bit faster than I thought," he told me. He counts himself doubly lucky that he did.
"My wife was across from the explosion," he said. "Once I passed her, she left the area."
Others weren't so lucky. At least three died. Some were maimed. Scores were injured. Coming in the midst of a celebrated public event, the attack was a grim reminder that the threat of terrorism isn't something we've put behind us.
Who, exactly, it was al-Qaida, a domestic terrorist like Timothy McVeigh, some other extremist group remains to be discovered.
But one way or another, it was obviously terrorism. The fact is, we aren't as safe as we had thought. Or hoped. So how will we respond?
Kwan was a first-time visitor to Boston. Will he come back?
"If I qualify to run," he said. "If I am scared of this, I guess I can't leave my house."
That's exactly the right attitude, for all of us.
Yes, we have to learn from this. And we will. We'll need to examine the security plan for the marathon and ask whether this could have been prevented. And we'll have to apply the lessons we learn to other public events.
But life here will go on. We won't be paralyzed by fear.
We'll take reasonable precautions, yes.
But we won't take cover.
And we won't cower.
This, after all, is Boston.
Reach Scot Lehigh at email@example.com.