The Boston Globe is reporting that police have intentionally set off another explosion in Boston “as part of bomb squad activities,” by the Boston Public Library (right near the original explosion).
Many have noted that today is Patriots’ Day in Massachusetts, a government and school holiday to commemorate the Battles of Lexington and Concord in 1775. The Marathon is an absolutely massive event, bringing half a million or more spectators to the city on Patriots’ Day Weekend; it’s probably the single biggest event in Boston each year.
The area where the first explosion occurred, the final stretch of the race and finish line (Boylston Street, in front of the Boston Public Libary) would not have been most crowded at around 3 p.m., when the explosion actually did happen, but around noon, when the elite runners finish and the stands are packed. The explosion occurred about four hours after most runners had begun the race.