Daisy Organ, head of the Child Abduction Section at the FCO, has told Daybreak that for parents whose children have been abducted to countries that are not members of the Hague Convention "face a long and distressing process to get them back."
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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.