After his capture by Boston police and federal agents, Dzhokhar Tsarnaev during his interrogation with FBI agents confessed that he and his brother, Tamerlan, had selected New York as a target to carry out their next attack. The brothers had intended to detonate explosives at Times Square.
Given that the brothers' actions killed three people and injured 264 others in Boston, had the two carried out their plan in New York, there is little doubt that the planned attack would have led to further casualties. If the brothers weren't stopped in Boston, who knows how much more damage they could have done?
The tragedy in Boston came as a shock given how long it has been since there has been a major attack in the United States. Since Sept. 11, 2001, there have been dozens of failed terror plots targeting U.S. soil, foiled often by law enforcement authorities and occasionally by sheer luck.