FBI agents arrive at the scene after explosions near the finish line of the Boston Marathon
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Who could be behind the Boston Marathon attack?

The FBI has launched a "potential terrorist inquiry" after two bombs at the Boston Marathon left three people dead and at least 140 injured.

They are asking runners and spectators to hand in their photos and video footage of the event.

One of the biggest challenges for agents is to find out if this latest terror attack was the work of domestic or foreign-backed extremists.

James Robbins reports.

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