Investigations were under way Tuesday into the twin explosions near the finish line of the Boston Marathon that killed three people and wounded more than 140.
The Federal Bureau of Investigation cautioned that "the situation remains fluid, and it remains too early to establish the cause and motivation."
Richard DesLauriers, the agent in charge of the FBI in Boston, had earlier said they were looking into a "potential terrorism investigation."
US President Barack Obama stopped short of calling the incident a terrorist attack. "We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts."
Security was increased in several major US cities, including New York and Washington, and organizers of upcoming marathons, including Sunday's race in London, were reviewing their security arrangements.
The two explosions took place within seconds of each other at 2:50 pm (1850 GMT) Monday near the finishing line, about two hours after the first runners had ended their race.
An 8-year old boy was among those killed, while 144 people were being treated in hospital, including at least 17 in critical condition and 25 in serious condition, CNN cited hospital officials saying.
The wounded including eight children. Ten people had lost limbs as the bombs scattered ball bearings, which at high speed cause extreme lower limb injuries, CNN reported.
As the powerful explosive devices went off, runners collapsed to the ground, glass and windows were shattered, spectators fanned out in panic, many of them bleeding and screaming, leaving pavements stained with blood.
Although the city would be largely open for business, armed police were to patrol Boston and search bags on public transport, the Boston Globe reported.
The crime scene around Copley Square would remain sealed-off. State troopers, National Guard units and municipal SWAT teams would be out in force.
Obama said Monday: "Make no mistake, we will get to the bottom of this. Any responsible individuals, any responsible groups, will feel the full weight of justice."
A White House official later said it was "clearly an act of terror," and "a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic."
About 26,000 runners participated in this year's Boston Marathon. First run in 1897, it is the world's oldest city marathon.
In contrast, the London Marathon is the world's largest, drawing more than half a million spectators from all over the world and passing by some of the capital's most well-known landmarks, including Buckingham Palace and Big Ben.
London Mayor Boris Johnson said: "We do have robust security measures in place for Sunday's London Marathon, but given events in Boston it's only prudent for the police and the organizers ... to re-examine those security arrangements."
International Olympic Committee President Jacques Rogge said: "A marathon is a special, almost magical event that unites different communities with one shared goal.
"And I am sure that this incident will serve to bring together all those who took part and all those who support such an event, with a common resolve."
Russian President Vladimir Putin called the attacks "a barbarian crime" and offered assistance to US investigators. In a telegram to Obama, he said the fight against terrorism needs to be actively coordinated worldwide.
Russian law enforcement agencies announced extra security measures for sports events, as the country is hosting the World Athletics Championships in August, the 2014 Winter Olympics and the 2018 football World Cup.
Security measures were also increased in France, with Interior Minister Manuel Valls calling for more frequent patrols by regional police. He said the public should avoid panic, but pay particular attention to suspicious objects.
President Francois Hollande sent his condolences to the victims and expressed France's solidarity with the United States.
EU foreign policy chief Catherine Ashton said "such acts are reprehensible," while NATO chief Anders Fogh Rasmussen offered his "heartfelt condolences to the families and loved ones of those who were killed."
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