President Barack Obama has condemned the bombings at the Boston Marathon as a "terrorist act."
He said the attack had been a "heinous and cowardly act," but said it was not yet known who carried out the attack and why.
The FBI has said there are "no known additional threats" beyond the two bombs which went off.
Three people were killed, including an 8-year-old boy, and more than 150 injured by the bombs.
The World's Arun Rath has been following the twists and turns of the investigation as details emerge.
And WGBH reporter Ibby Caputo has been making the rounds of Boston hospitals, finding out how doctors and nurses are faring, treating those who were wounded by Monday's bomb blasts.