Via WSJ:

Two deadly explosions ripped through a crowd at the Boston Marathon on Monday, killing at least two people and injuring more than 110, 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 were still on the streets, two bombs exploded near the course’s end in the city’s crowded Back Bay section, Boston Police said.

Panicked spectators fled the scene as smoke from the blasts filled the air, sirens wailed and exhausted, disoriented runners and families raced to find one another amid the mayhem.

Counterterrorism officials found what they believe to be five additional, undetonated explosive devices around the Boston area, according to two people briefed on the rapidly moving investigation.

Update:

(TPM) — After initially reporting that authorities found five additional undetonated devices in Boston, the Wall Street Journal now says officials doubt they were indeed bombs:

Two people briefed on the investigation said officials initially believed that the five additional devices could be undetonated explosives. Later Monday, a law-enforcement official said subsequent closer examinations led investigators to doubt that they were bombs.