Iowa's six electoral college votes have gone to Barack Obama, according to NBC News projections.
The President now leads Mitt Romney by 249 to 203, with Florida and Ohio yet to be called.
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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.