An investigation is under way after two explosions near the finishing line of the Boston Marathon left three people dead and at least 140 injured.
The FBI has taken over co-ordination of what it described as a "potential terrorist inquiry".
Boston police say their officers are working around the clock and all leave has been cancelled.
No group has said it carried out the attack, which left at least 17 people critically wounded, officials say.
One of the dead was an eight-year-old boy, US media say, while several of those injured have had to have limbs amputated.