FBI Special Agent Rick DesLauriers told reporters in the aftermath of Monday's attack that while investigators were fanning out across Boston, there was "no known imminent physical threat at any location" in the area.
"It will be a worldwide investigation. We'll go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime and we will do everything we can to bring them to justice," DesLauriers said.
The agent also said the US Federal Bureau of Investigation was "not aware of any threat information prior to the marathon."
Officials put the new toll from the attacks in the northeastern US city at three dead and more than 170 wounded, many of them with traumatic leg injuries, indicating the explosive devices were placed low to the ground.
Massachusetts Governor Deval Patrick said only two bombs were used in Monday's carnage near the finish line of the marathon, and no other other unexploded devices had been found despite earlier reports to the contrary.
"It's important to clarify that two and only two explosive devices were found yesterday," Patrick said.
"Other parcels - all other parcels in the area of the blast have been examined. No unexploded bombs. No unexploded explosive devices (were) found."
"Our thoughts go out to all of those injured and killed," he said.