It's a scenario that plays out with all too frightening regularity in the United States.
People gather to enjoy an event or a night out. The good times are shattered in a split second by violence. It would seem that the only difference between these occurrences are the number of victims.
As the government is pushed one way or another regarding guns in the wake of the Connecticut school shooting and the Colorado theater shooting, someone makes some bombs and sets them off near the finish line of the Boston Marathon.
And while the national debate about violence will continue, as it has seemingly nonstop since two teenagers shot up Columbine High School in suburban Denver 14 years ago - the incidents continue.
It's not possible to harden every place in the nation that presents a target to a gunman with a mental defect or a bomber with an ideology. Would Americans want to live in a nation with the Army patrolling the streets instead of just the airports at times of heightened alert? Would Americans tolerate being searched before entering a shopping center, or to go through more extensive security checks when entering a major sporting event?
Would Americans use the violence as an excuse to stop gathering?
We think the answer to all of those is "no."
That means we have to watch out for ourselves.
It's a given that any abandoned bag in an airport is worthy of suspicion, and most travelers know to report any such bags to security. Announcements in the airports drill that knowledge home.
But how aware of we of our surroundings when we're not in the air travel system?
Homeland Security officials have issued a reminder this week that people should be wary of unusual activities, including:
Being aware of cars and trucks entering and leaving familiar parking areas.
Watching for people with unusual stains on their clothing or glimpses of wiring on their person
People with unusual or prolonged interest about security cameras, access controls and security arrangements.
Further, be aware of people who might just be casing security responses by leaving packages or suitcases or other items in sensitive or vulnerable areas.
And it goes without saying that any vague threat about harm to persons or places must be taken seriously nowadays.
We hope the Department of Homeland Security makes a national public education effort at driving those points home to every citizen.
Somewhere, someone heard or saw something that connects the Boston Marathon bombs to the coward or cowards who built and detonated them.
It only takes a little suspicion ahead of time to slow, and hopefully stop, the ongoing roll of violence.