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The Boston Marathon is one of the world’s oldest and most prestigious races and about 23,000 runners participated. Most of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course.

The attack may have been timed for maximum bloodshed: 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 friends and relatives clustered around to cheer them on.

Davis, the police commissioner, said authorities had received “no specific intelligence that anything was going to happen” at the race. On Tuesday, he said that two security sweeps of the route had been conducted before the marathon.

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.

Richard Barrett, the former U.N. coordinator for an al-Qaida and Taliban monitoring team who has also worked for British intelligence, said the relatively small size of the devices in Boston and the timing of the blasts suggest a domestic attack rather than an al-Qaida-inspired one.

“This happened on Patriots Day — it is also the day Americans are supposed to have their taxes in — and Boston is quite a symbolic city,” said Barrett, now senior director at the Qatar International Academy for Security Studies.

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Eileen Sullivan contributed to this story from Washington. Associated Press writers Jay Lindsay, Denise Lavoie, Steve LeBlanc, Bridget Murphy, Rodrique Ngowi and Meghan Barr in Boston; Julie Pace and Lara Jakes in Washington; Paisley Dodds in London and Marilynn Marchione in Milwaukee also contributed.