The two blasts occurred within seconds at a sidewalk along the route of the Boston Marathon, where thousands of people had lined up to cheer on the marathoners.
The Boston Globe quoted law enforcement sources as saying that the dead included an 8-year-old boy. The injured are being treated at several local hospitals.
The authorities also found a device at two other locations in the city, but they did not explode.
FBI special agent in charge Richard DesLauriers told reporters here that the investigation into the blasts is an "ongoing and criminal investigation that is a potential terrorist investigation."
After the incident, the security across major US cities was beefed up and President Obama, describing the act as "senseless", affirmed that the perpetrators "will feel the full weight of justice."
Nearly 27,000 people were participating in the marathon which is held annually on the Patriots' Day, the third Monday of April, a state holiday in Massachusetts.
The blasts occurred as thousands of spectators lined the sidewalks to cheer on the marathoners. several runners collapsed on the ground due to the impact of the blasts which tore through the windows of buildings nearby.
President Obama said those responsible are yet to be identified, and cautioned that "people should not jump to conclusions before we have all the facts."
"But make no mistake, we will get to the bottom of this.
And we will find out who did this and we'll find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice," Obama said.
Obama did not say whether the blasts were a terror attack.