An explosion at a small factory in a Mumbai slum on Friday killed five people and injured three others when the wall of a neighbouring house collapsed, police said. Forensic investigations are under way to clarify the cause of the blast, which happened before dawn in the city\'s Andheri suburb. \"There was a small explosion at a makeshift industrial unit, which caused a slab of wall in an adjacent house in the slum to collapse, killing those who were asleep,\" a police official in the city control room told AFP, declining to be named. The injured are being treated at nearby hospitals, he said. \"We came here after hearing people crying for help. We helped an old man escape,\" eyewitness Sartaj Ahmed told the NDTV news channel, which described the factory as a \"chemical producing unit\". \"We came out and saw a lot of smoke,\" said another local resident woken up by the blast. More than half of Mumbai\'s residents live in tightly-packed slums in the city.