Washington, Apr 16: US President Barack Obama said on Monday that authorities "will find out who did this and we will hold them accountable" after two bombs exploded near the finish line of the Boston Marathon, killing three people and injuring more than 130 others.
Obama urged a nervous nation to avoid jumping to conclusions while acknowledging that officials "still do not know who did this or why."
A White House official later said the incident was being treated as "terrorism".
Speaking from the White House just three hours after the explosions, Obama was cautious in his remarks, stopping short of
calling the incident an act of terror.
Obama said the explosions were the act of an individual or group, but vowed to hold those responsible accountable.
"We will find out who did this. We'll find out why they did this," Obama said in his brief, three-minute statement.
"Any responsible individuals, any responsible groups, will feel the full weight of justice."
The president said the government would increase security around the United States as necessary but did not say whether his administration thought the incident was part of a larger plot.
A senior U.S. intelligence official said two other explosive devices were found near the end of the 26.2-mile (42-kilometre) course.
The president was briefed on the incident Monday by several senior administration officials, including FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano.
He also spoke with Massachusetts Governor Deval Patrick and Boston Mayor Tom Menino and pledged to provide whatever federal support was needed in responding to the incident.
Additionally, the president spoke with Republican and Democratic leaders on Capitol Hill, saying that "on days like this, there are no Republicans or Democrats, we are Americans united in our concern for our fellow citizens."
The twin blasts at the race took place almost simultaneously and about 100 yards (90 metres) apart, knocking spectators and at least one runner off their feet, shattering windows and sending smoke rising over the street.
Some 23-thousand runners took part in the race, which winds up in the heart of central Boston near Copley Square, near the Prudential Tower and the Boston Public Library.
The Federal Aviation Administration barred low-flying aircraft from within 3.5 miles (5.6 kilometres) of the site.
A fire broke out at the John F. Kennedy Library a few miles (kilometres) away.
The police commissioner said it may have been caused by an incendiary device but it didn't appear to be related to the bombings.
