At least 20 people were killed and 21 others injured when a portion of a five-story building collapsed after heavy monsoon rains in downtown Mumbai, police said Wednesday. "There could be more people trapped under the debris," said police officer Dheeraj Chauhan, adding that the torrential rains were hampering rescue efforts. Police said 11 people were killed at the scene while nine others who were critically injured died later at hospitals, most from serious head injuries. The building was located in a crowded residential neighborhood with narrow lanes, making it difficult for rescue workers to provide quick medical treatment at the site, Chauhan said. Firefighters struggled along with volunteers to sift through mounds of concrete beams and smashed wooden furniture. Police estimate the dilapidated building was around 65 years old.