SHANGHAI - Five people suffocated in their sleep due to natural gas poisoning in the eastern Chinese metropolis of Shanghai Tuesday night, local police said Wednesday.

Local police were called by tenants of a residential building in Fengxian district who reported smelling gas at around 10 pm on Tuesday. The police found that five people living in an apartment had suffocated to death in their sleep, the district police said.

An initial investigation shows that Li Yonggang, a man in his 20s from Sichuan province, had kept tropical fish for sale in the apartment and used natural gas to keep the fish warm. The combustion of natural gas suffocated the five people living in the 120-square-meter apartment.