The brothers suspected of the Boston Marathon bombings originally planned to carry out the attack on July 4 this year, a law enforcement official has said.
But the official claimed the Tsarnaev brothers decided to strike earlier because they completed building their bombs more quickly than expected.
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A shock to the system
We found a tableau of desperation; vast crowds of people pushing and shoving their way forward, away from the continuing bombardment.
Cambridge Analytica played a key role in mapping behaviour of voters in the US election and was used during the EU referendum campaign.