Ten seconds and 100 yards. The time and distance between two bombs.
Two crudely built bombs set off 10 seconds and 100 yards apart without warning or threat, ripping through a crowd of spectators and runners, filling the street with fire, blood and limbs. Two bombs that began a cascade of casualties and terror, that triggered a massive manhunt that paralyzed a city.
It began Monday with two bombs 10 seconds and 100 yards apart just shy of the finish line of the Boston Marathon, blasts that sounded the start of a new race -- to identify and find those responsible. This is how that race unfolded.
MONDAY, APRIL 15
-- Just before 3 p.m., an explosion shatters the cheers on Boylston Street near the finish line of one of Boston's largest and most cherished events. More than 17,000 runners already had crossed the finish line, but thousands more still were headed for the site of the bombing. Ten seconds later, a second explosion shatters windows and bodies. Sirens and screams erupt as rescuers scramble and the crowd panics.
"They just started bringing people in with no limbs," runner Tim Davey of Richmond, Va., said of his view from inside a medical tent that had been set up to care for fatigued runners.
-- The blasts killed three people -- 8-year-old Martin Richard, of Boston; 29-year-old Krystle Campbell, of Medford; and 23-year-old Lu Lingzi, a Boston University graduate student from China -- and injured more than 180 others, but it would be hours before the chaos cleared enough to give authorities a true sense of the casualties. Or even where the bombs had been hidden. If they had been hidden.
-- A citywide shutdown that will become nearly complete by the end of the week begins by early evening. A no-fly zone is created over the bombing sites, major sporting events are canceled, people are urged to stay indoors, SWAT team members with machine guns patrol hospitals. And the world takes notice, beefing up security at nuclear plants, public transit systems and anywhere crowds gather.
-- Is it terrorism? Americans are eager for answers, but when President Barack Obama addresses the nation three hours after the explosion, he stops short of that. "We will find out who did this. We'll find out why they did this," Obama said in his brief statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
-- Knowing that thousands of smartphones and cameras were in the crowd, by nightfall authorities officially tap the power of crowd sourcing and put out the call for pictures, videos and tips.