The FBI said it had launched a "worldwide" investigation into a terrorist attack at yesterday's Boston Marathon and called for public help in identifying the perpetrators.
US authorities urged people to submit any photographic and video material taken at the scene of the twin blast which killed at least three people, including an eight-year-old child, to determine exactly who was in the area at the time.
"This will be a worldwide investigation. We will go to the ends of the earth to identify the suspects responsible for this despicable crime," said FBI special agent in charge of the investigation, Richard Deslauriers. No arrests have been made yet.
At a press briefing in Boston, Deslauriers said authorities received "voluminous tips" in relation to the bombing, which are being investigated. No details on the nature of the explosive devices used by the terrorists were provided.
"This is an active and ongoing investigation. I ask for your patience and understanding as we continue to work to get at the bottom of who did this and why," said US attorney Carmen Ortiz.
No group has yet claimed responsibility for the bombings, which took place near the finish line of the marathon and injured 176 people, 17 critically.
The Pakistani Taliban, which has a history of terrorist threats against the US, denied any responsibility for the twin blasts.
Two bombs went off near the end of the 26.2-mile (42km) course, injuring some of the 23,000 runners and more than 500,000 spectators watching the race.
Eight-year-old Martin Richard has been named as the youngest victim of the attack that took place on one of the Boston's most famous civic holidays, Patriots Day.
"So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely," said a European security official, who spoke from the US on condition of anonymity.
Boston police commissioner Ed Davis said authorities believe the three people who died in the blast are all victims of the attack and not perpetrators.
"We are securing and processing the most complex crime scene we have had to deal with in the history of our department," Davis said.
Massachusetts governor Patrick Deval said authorities are "working around the clock" to find leads in the bombing he defined as an "act of terror".
"The state will be open for business today, but it will not be business as usual," Deval wrote in the statement announcing additional security measures have been taken across the eastern US state.
Deval clarified early reports of several unexploded bombs discovered around Boston, saying that only two explosive devices had been found. Other parcels left at the scene have been examined and some suspect packages were detonated but they didn't contain any bombs.
"It is a bad day for Boston but if we pull together we will get through it. Boston will overcome," city mayor Thomas Menino said.