Osvaldo Gonzalez and Alberto Arias, friends and business partners, happened to pass under a Miami bridge that Thursday afternoon, the road bustling with fellow drivers also out on the most ordinary and unthreatening of life's tasks.

A teenager was driving her friend to the doctor's office to pick up some medicine. A father of three was heading home from work. A woman on her way to a nail salon was stopped at a red light. Seconds — inches — would soon separate those who would live from those who wouldn't.

Sweetwater police Detective Juan Llera was at his office a few blocks away, when he heard what he thought was a bomb exploding. ...