The bomb ripped through the crowd gathered on a Boston sidewalk, producing a flash of light and then a haunting yellow plume of smoke which immediately blew through a row of flags.
There's today's image of terror: screams and carnage and smoke and flags.
Flags representing countries from around the world, flags lined up on Boylston Street because athletes from all these nations come every April to compete in a special race, in front of special fans, to celebrate two special things – Patriots Day and the concept of individual accomplishment, often against great odds.
Boston has never been one to back down from a fight, not at dawn on Lexington Green in 1775 and assuredly not on Monday when the annual commemoration – a race, a Sox game, a hope for a speck of spring weather – was torn apart the way too many things are these days.
Senseless violence. Cowardly acts. Innocent casualties. A murdered 8-year-old. Blood staining Boylston at the finish line of the Boston damn Marathon.
Only, as too often is the case now, no one knows who was trying to fight, let alone what it was all about.
"We still do not know who did this, or why," President Obama said. Those questions will no doubt soon be answered and retribution will no doubt be significant.
For the families who lost loved ones and the injured facing a lifetime of recovery, the impact will be impossibly severe. There aren't adequate words to convey the rest of our thoughts and prayers and pain. Helping them is the only important goal.
For everyone else who once again watched in helpless horror from afar, trying to make sense of the smoke curling through the flags in another tragic moment in America – where do we go?
It almost doesn't matter who the killers were, only that there was killing once again. Their motives deserve no special attention. Their acts are no different than too many other days when civility is shredded.
Monday it was in the Back Bay, but this was the same shaky video of a truck bomb smashing through Oklahoma City or planes plowing into the World Trade Center or a blast one night at the Olympics in Atlanta or some coward walking into a movie theater or an elementary school and opening fire.
The scenes change but almost nothing else does.
Horror and heroes. Tears and torment. Questions and more questions and never an answer.
This time, someone went after the Boston Marathon, one of the biggest events of its kind with upward of half a million fans, and one of the softest targets. It's a public 26.2-mile course winding from the countryside out in Hopkinton and through six additional towns before rolling into the heart of a major city. The perpetrators apparently hid bombs in garbage cans and ran.
How do you guard all of that, against all of that? About the only surprise is that whoever wanted to do this didn't do it sooner.
This was no random target. They rarely are.
This is a one-of-a-kind event, a one-of-a-kind day, which speaks to all those flags. There are marathons and there is Boston. The course isn't fast because there are too many hills. The date isn't convenient; you need to train through the winter. The weather isn't good; sometimes it's downright brutal.
Yet every year the world arrives. And every year the fans come out and manage to turn it into a treasured spectacle. All these people stand and cheer a marathon – about the dullest imaginable sport to watch in person.
The terrorists knew that. The bombs went off four hours, nine minutes and 43 seconds after the race started, two hours after the leaders blazed through the finish line. Only in Boston would there still be a crowd clapping and screaming for random plodders. This is the people's race and it's been that way since 1897.
There really are two Boston Marathons. One is for the professionals, the best of the world, who arrive, mostly from Kenya and Ethiopia, to compete for prize money.
Then there is everyone else: the highly competitive and cardio-addicted, the naturally gifted and relentlessly relentless. There are runners raising money for cancer and runners celebrating their defeat of the disease.
There are those who use the idea of running down Boylston as the carrot to turn their life around, running toward a new day or from an old one, knowing their proud friends and families would be waiting for them to celebrate.
Boston isn't a symbol so much as the destination. It isn't about hope. It's about accomplishment. There is exhaustion at the finish. There is also emotion and elation. This is about pushing to the limit and seeing the other side.
And for whatever reason – be it habit or holiday – the people of Massachusetts come each year to honor that, to root on the anonymous man or woman hammering up Heartbreak Hill, everyone recognizing an individual story of great accomplishment, even if they don't know the details.
Boston is a concept that crosses all races and religions, all generations and geography. This is the world's finish line. That's why the row of flags was there. The blast almost knocked a couple over, only to have them stay aloft. Three yellow balloons did come undone, floating gently up as a symbol of stark contrast to the hell raging below inside that smoke.
In videos of the explosion – on the other side of Boylston – is the sight of an adult pushing a child in a racing wheelchair. The blast nearly toppled the chair.
They are believed to be part "Team Hoyt", a concept and eventual worldwide phenomenon founded by Dick and Rick Hoyt over three decades ago here.
In 1977, Dick began pushing his son, Rick, who suffers from cerebral palsy, as a way to help connect with each other. The doctors had said Rick should be institutionalized. Dick would have none of it. They started with small races around New England at a time when there were no divisions for people like them. They eventually set their sites on the Marathon.
The sight of the two of them – Dick hunched over pushing hard, Rick up front smiling and waving to the crowds – started in 1981 and became an annual vision. Their story began to be told. First locally, then nationally and, finally, everywhere. Team Hoyt became heroes. They raised millions, inspired millions more and continue to serve as profound examples for families dealing with severe physical and mental issues.
On Monday, Dick, now 73, and Rick, 51, ran their 31st Boston Marathon and, according to their official time, finished well before the blast. The wheelchair entry in the video is their legacy, though – either directly through their team or indirectly by their use of this famous race to change the way the world works.
Last week, bronze statues of the Hoyts were placed in Hopkinton, alongside former champions. It was the ultimate sign of what this race, this day, this entire phenomenon is about. Promises kept, dedication on display, how a single-minded focus can make the seemingly impossible occur.
Here on a day officially about remembering an old battle that helped start a democracy – a day with a soundtrack of cheers for individual accomplishments won a single step at a time – the world's flags flapped over the figure of someone pushing a kid in a wheelchair. That kid was once inspired, and maybe now can inspire, too.
Forget, for a moment, the who and the why. Instead think about the dead and wounded. And then think about the where and when those bombs went off. That moment right there, a crowded city street full of flags and wheelchairs and accomplishments.
That's what someone tried to destroy on Monday. That's what they wanted to kill.
Yet that's also what they never, ever will.
More Boston Marathon explosions coverage on Yahoo!:
• Twitter reaction to the Boston Marathon explosions
• Celtics game canceled, not postponed, after Boston Marathon bombing
• Y! Shine: How you can help and stay informed
- Sports & Recreation
- Boston Marathon