Specialist police divers are searching a lake near the spot where officers found a missing professional golfer's car.
Peter Jackson, 38, has not been seen since he left his house in Seaham on Monday, 11 November.
His car was found in a car park at Cawfield Quarry, near Haltwhistle, on Tuesday night.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
Newcastle Central Mosque is welcoming Muslims and non-Muslims alike this evening to stand in solidarity with the people of Manchester.