It was as good a Patriots Day, as good a marathon day, as any, dry and seasonably warm but not hot like last year's. While the runners climbed Heartbreak Hill, the Red Sox were locked in another white-knuckle duel with the Tampa Bay Rays at Fenway Park. The only thing missing was Lou Reed crooning Perfect Day.

The elite runners had long ago finished when Boston won the game 3-2. Many of those jubilant baseball fans had walked through Kenmore Square toward Back Bay to watch the marathon. Some of them had just gotten to the finish line when the first bomb went off.

In an instant, a perfect day had morphed into something viscerally evil.

The location and timing of the bombs was sinister beyond belief, done purposely to maximize death and destruction. Among those who watched in horror as a fireball belched out across the sidewalk on Boylston Street were parents of the schoolchildren murdered in Newtown, Connecticut.

One of the dead is an 8-year-old boy from Dorchester who had gone out to hug his dad after he crossed the finish line. The dad walked on, and the boy went back to the sidewalk to join his mom and his little sister. Then the bomb went off. The boy was killed. His sister's leg was blown off. His mother was badly injured. That's just one family, one story.

It would be wrong to say we lost our innocence on Monday afternoon as a plume of white smoke drifted high above Boylston Street, as blood pooled on the sidewalk across from the Boston Public Library. Severed limbs lay amid the bruised and the bloodied and the stunned, their ears ringing, their ears bleeding.

We lost our innocence on another perfect day, in September, 12 years ago. But we lost something on Monday, too, and that is the idea that we will ever feel safe in this city again.

The Boston Marathon is the city's signature event, a tangible link with the rest of the world. It allows us to cling to that pretense of Boston being the Hub of the Universe. Patriots Day is a celebration of our revolutionary history, but we share it with the world. It is the one day of the year when the city is its most diverse, with people from so many other countries here to run those 42 kilometers.

And so it was alternately poignant and horrifying to watch as first responders frantically pulled metal barriers and the flags of so many different countries down in a desperate rush to get to the dead and the injured on the sidewalk. Those flags looked like victims, splayed on Boylston Street as the acrid smoke hung in the air.

Everybody was trying to figure out who and why. The police officers I talked to were shaking their heads. It could be anybody. Could be foreign. Could be domestic. Could be al-Qaida. Could be homegrown nuts.

It was Patriots Day. It was tax day. It was Israel's independence day. Theories swirled like the smoke above Boylston Street. Friday marks the 20th anniversary of the FBI assault on the Branch Davidian compound in Waco, Texas, and the 18th anniversary of the Oklahoma City bombing.

In an instant, the medical tent at the finish line was transformed into a battlefield triage unit. Doctors and nurses who had been running the race in turn raced to the medical tent and volunteered their services, still sweating, still wearing their running gear. People opened their homes to runners who could not get back to their hotels. Even as the smoke drifted away from Boylston, we were still in the fog, still in the dark, our ears still ringing from the bombs.

Kevin Cullen is a Boston-based journalist.