BOSTON—Two deadly explosions ripped through a crowd at the Boston Marathon on Monday, killing at least three people and injuring about 140, once again raising the specter of terrorism on American soil.

At about 2:50 p.m., some three hours after the winners had crossed the finish line but while thousands more runners still were on the streets, two bombs exploded 50 to 100 yards apart near the course's end in the city's crowded Back Bay section, Boston Police said.

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