Downing Street has said there are “questions to answer” after a fire at an electrical substation closed down London Heathrow airport, stopping more than 1,350 flights and disrupting the journeys of hundreds of thousands of passengers. Counter-terrorism police are leading the investigation into the “unprecedented” incident that left Britain’s biggest airport unable to function as engineers tried to restore power, but said there was “no indication of foul play”. Two people familiar with the investigation said officials did not believe the fire was the result of any criminal activity or a hostile state and was more likely to be accidental.