At least four people, one of them a seven-year-old child, have been killed when the building they were in collapsed in the northern Italian city of Milan. Some 50 people were also injured in the blast, which firefighters say was probably the result of a gas leak. The explosion happened in an apartment block at 6 pm local time. The young son of a bar owner whose premises were on the ground floor of the building is one of the fatalities. The others are a man in his sixties who lived at the address, a younger male passer-by and a woman in her fifties who was found amid the rubble of her apartment in the early hours.