Witnesses said the first blast went off at the Boylston Street finish line as spectators were cheering on people about three hours into the 117th running of the 26.2-mile (42.19-kilometer) race on Monday. It was followed by a second explosion just seconds later.
Smoke rose 50 feet in the air, and people began running and screaming after hearing the noise. Video images showed bloody spectators looking dazed and chaos broke out. Ambulances and firefighters poured into the scene, rushing the victims to hospitals.
While reports continue to emerge from the scene, a number of facts point toward the typical setup used to devise, control, and execute such an attack as took place around the 9/11, 7/7, and OKC bombings as well as the Aurora and Sandy Hook shootings.