Marcus Powell spoke to a local resident who saw a police officer being attacked by the dog in East London.
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He told her "If you don't be quiet I will kill you" before stealing a bangle and rings from her hand.
One of the alleged kidnappers of model Chloe Ayling is fighting extradition as his lawyer claims the entire case could be a "sham".
Uber's chief executive Dara Khosrowshahi said the company "got things wrong".