The second explosion could be heard a few seconds later.They occurred about four hours into the race and two hours after the men's winner crossed the line. By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.The attack may have been timed for maximum carnage: The fourhour mark is typically a crowded time near the finish line because of the slowbutsteady recreational runners completing the race and because of all the relatives and friends clustered around to cheer them on.Runners in the medical tent for treatment of dehydration or other racerelated ills were pushed out to make room for victims of the bombing.A woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up.