A TV grab shows people at the finish line of the 2013 Boston Marathon following an explosion in Boston.
Two explosions at the finishing line of the historic Boston marathon on Monday killed at least two people and injured 46. There is no information yet on who was behind them.
The explosions, which were described as "loud canon-like booms" that sent up plumes of smoke and dust, took place as some runners were crossing the finishing line.
A third explosion took place at another location, JFK library, away from the marathon route. A fourth explosion was a device denoted by authorities shortly after discovery.
There were 15 Indian nationals among the runners. Some of them had completed the race and would have been away from the finishing line when the explosions took place.
"We dont know who did it but make no mistake we will get to the bottom of this," President Barack Obama said in a grim three-minute address from the White House.
He did not use the word "terror" even once.
Still image taken from video courtesy of NBC shows an explosion at the Boston Marathon. Reuters
White House officials said Obama had been informed of the explosions and he had ordered federal authorities to help in rescue and investigation.
The number of dead stays at two, with the injured said to be between 80 and 100, the latter figure coming from the Boston Globe, a local newspaper.
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The police refused to either give out the exact extent of casualties saying it was too early to say. And they also didn't confirm or them as the handiwork of terrorists.
"It's too early to say," said Boston Police commissioner Ed Davis at a news briefing, when asked if the incidents were terrorists attacks.
But investigator were treated the explosions as linked.
Though there have been many attempts, the US has not had a terrorist attack since the September 11, 2001 incidents in New York city and Washington DC suburbs.
Washington DC and New York city went on an alert as a precautionary measure, with tourists pushed to a safe distance from the White House, a routine security procedure.
A woman comforts another, who appears to have suffered an injury to her hand, after explosions interrupted the 117th Boston Marathon in Boston. Reuters
Boston marathon is an annual event with runners from all over the world. Security and medial arrangements are always intense, which may have helped in the injured reach hospital promptly. Factbox: Key facts about the Boston Marathon
Explosion took place around 2:50pm Eastern time (12:20am IST) the first deaths were confirmed by Boston Police around 4:05pm (1:35am IST).