MUMBAI (AFP) - At least 20 people died and 21 others were injured Wednesday when part of a four-storey building collapsed in India's financial capital Mumbai, police said. "The back portion of the building, which was over 60 years old, collapsed suddenly when people were asleep," a police official told AFP. "The death toll stands at 20 so far" from the pre-dawn collapse, he said. Fire brigade workers and volunteers were forced to dig through the debris with shovels and bare hands in a frantic search for survivors, as the bylane where the building was located was too narrow to bring in heavy equipment. Police said more victims could still be trapped under the rubble. The Chief Minister of Maharashtra state, Vilasrao Deshmukh, visited the site, and promised an investigation into the cause of the building's collapse. Such incidents are common in India where dwellings often collapse due to poor construction materials, lack of building regulations and heavy rains. Mumbai has been experiencing heavy monsoon rains, weakening flimsy structures.