Competitors and race volunteers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners, and Massachusetts General Hospital said it had received four patients and was expecting more.
About three hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.
Smoke rose from the blasts, fluttering through the national flags lining the route of the world's oldest and most prestigious marathon. TV helicopter footage showed blood staining the pavement in the popular shopping and tourist area known as the Back Bay, and the windows were blown out of multiple storefronts.
A bomb squad was en route to the site of the blasts, NBC News reported. The cause of the blasts remained unclear.
UPDATES AND COMPLETE COVERAGE AT NBCCONNECTICUT.COM