The challenge of securing the Games was made clear the day after the U.K. won its Olympic bid: On July 7, 2005, a series of coordinated suicide bomb attacks in London killed 56 people and injured more than 700.

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India's home secretary has said Wednesday's series of bomb blasts in the western city of Pune were a "planned and coordinated" act.

BBC: India: Pune blasts 'planned and coordinated'