US president Barack Obama said those responsible for the attacks feel the "full weight of justice".
The US president told a news conference at the White House: "The American people will say a prayer for Boston tonight, and Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss.
“We don’t yet have all the answers but we do know that multiple people have been wounded, some gravely, in explosions at the Boston Marathon.”
He added: “We salute all those who assisted in responding so quickly and professionally to this tragedy.
“We still do not know who did this, or why. And people shouldn’t 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, we will find out why they did this.
“Any responsible individuals, any responsible groups, will feel the full weight of justice.”
Earlier Mr Obama was briefed following the explosion by FBI director Bob Mueller and Homeland Security secretary Janet Napolitano.
A White House official said Mr Obama was told about the "active investigation and response to the incident in Boston" as well as the ongoing coordination with state and local officials.
Reuters