German candidate Angela Merkel has cast her ballot in an election that is widely expected to hand her a third term in office.
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Mohanna Abdhou, known as Montana, was out with friends when two attackers cycled up to them and opened fire in Brent, north-west London.
The US president tweeted that Mrs May gave him "full details" of the incident during talks while he attended summits in Europe.
Police have raided another home in connection with the Manchester Arena attack as officers made a fresh arrest as part of the investigation.