New York Mayor Michael Bloomberg and the city's police commissioner Ray Kelly are speaking in response to claims the Boston Marathon bombing suspects planned to detonate bombs in Times Square.
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The prime minister was shouted at as she left the scene of the tragedy in which at least 79 people died.
Father-of-six Makram Ali, 51, was already receiving medical treatment as the attack unfolded. He died from multiple injuries, police say.
A review of how police and security services dealt with the latest terror attacks in the UK is to be launched, Amber Rudd has announced.