The FBI has taken over co-ordination of what it described as a "potential terrorist inquiry²".

 

Boston police say officers are working around the clock and all leave has been cancelled.

 

At least 17 people are critically wounded, officials say, and the injuries include several amputations（截肢）.

 

One of the dead was an eight-year-old boy, US media said.

 

In a televised address, President Barack Obama vowed³ to bring those behind the attack to justice.

 

"We will find out who did this. We'll find out why they did this," he said.

 

"Any responsible individuals, any responsible groups, will feel the full weight of justice."

 

Although he did not use the word "terrorism", a White House official later said: "Any event with multiple explosive devices - as this appears to be - is clearly an act of terror, and will be approached as an act of terror."

 

The first explosion came at about 14:50 local time (18:50 GMT) on the north side of Boylston Street, about two hours after the winners crossed the line.

 

There was initial confusion and panic. Some runners fell to the floor while police and bystanders ran to help those caught in the blast.

 

Then seconds later, another explosion ripped into the crowd further away from the finishing line, between 50 and 100 metres (yards) from the first blast. 

 

TV footage showed bloodied⁴ runners and spectators being treated at the scene and the road strewn with debris⁵. Rescuers tore down temporary fencing to reach the casualties.

 

Medical officials speak of at least 140 injured, with injuries including severed⁶ limbs, shrapnel wounds, cuts and bruises, 

 

At an initial news conference, Boston Police Commissioner⁷ Edward Davis said authorities had received "no specific intelligence that anything was going to happen".

 

He urged people to stay indoors and not congregate⁸ in large groups.

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