"We still don't know who did this or why," Obama said at the White House, adding, "Make no mistake: We will get to the bottom of this."
Boston hospitals that were prepared to treat injuries from a rigorous road race instead mobilized disaster plans to treat the more than 140 people injured. The victims' injuries included broken bones, shrapnel wounds and ruptured eardrums. At Massachusetts General Hospital, Alisdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
One of Boston's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775. The Red Sox always play their home opener on Patriots Day and schedule it so the game ends in time for fans to leave Fenway Park and cheer on the more than 23,000 runners in one of the world's oldest and most prestigious marathons.
The bombings 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 more were still running.
The attack may have been timed for maximum carnage: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the relatives and friends clustered around to cheer them on.
Glenn Herlihy of Haverhill was standing just in front of the Lenox Hotel on Boylston Street waiting for his brother Mark to finish his first Boston Marathon. Mark Herlihy was expected at any minute when the first bomb exploded to Glenn Herlihy's right. Before he could process what had happened, the second went off to his left. He was caught right in between the twin blasts.