By Chad Finn, Boston.com Columnist

It sounded like thunder, improbable in retrospect on what was such a spotless New England spring day.

But other explanations for the single burst of noise that briefly rattled the Boston Marathon media center at the Fairmont Copley Plaza hotel were plausible enough that fear of the worst was momentarily fleeting:

"That sounded like a bomb,'' I thought. Then, having filed my story on the elite race winners, I went to get a cup of coffee. It was 2:52 p.m.

I was two steps out of the ballroom that serves as the media nerve center when I ran into colleague Eric Wilbur. He had just been out on Boylston Street, talking to finishers for his assignment. Blood was absent from his face.

"Did you hear that? That was a bomb,'' he said. "Two of them. Seconds apart. This is serious [expletive]. It's chaos."

I ran back to my station to collect my phone and notepad, but it was already too late.

We were immediately put in lockdown. No one was allowed to leave the hotel. National Guardsmen soon took posts at the doors.

But those on the outside -- reporters, volunteers, runners and their families and friends -- entered in search of shelter from the scene, gradually filling the long hallways.

On a couch in the lobby, Joan Benoit Samuelson, who just a little more than an hour before had achieved her goal of finishing the race in 2 hours and 50 minutes, sat on a couch as a friend consoled her. "I'm shaken but OK,'' she said. "I just hope everyone outside is.''

The gravity of the situation became apparent from the fear on the faces of those who had come in from outside and the gruesome stories they brought with them.

A colleague who had been no more than 20 yards from the first explosion arrived in the media center, tears welling in his eyes as he spoke of witnessing a man lose a leg at the knee.

A volunteer said lanyards were being used by first responders as tourniquets.

The horrific anecdotes were soon accompanied by numbers. Two deaths. At least 28 injured. A strange incident at the JFK Library, and reports of other suspicious devices around the city.

Context may be within reach today for some, but damned if I can find it. We don't know who did this. We don't know if it was a sinister mastermind or a random lunatic.

We don't know if we're safe, or if there is more to come. We don't know.

What we do know is that he or she or they knew what precisely they were doing, their act of terror timed for significant chaos and casualties.

We do know that Patriots' Day will forever be accompanied by solemnity and sadness, a reminder that this happened in our city, on our uniquely Boston day.

And know this: That every clap of thunder will now bring unease and fear, leaving us to quickly pray and plead that we're not about to underestimate the devastation just outside the door.