Three workers were killed when a huge water tank under construction for a sewage treatment plant collapsed in the northern suburb of Bengaluru in Karnataka on Monday, police said."The centering material of the tank, being built by the state-run Bangalore Water Supply and Sewerage Board (BWSSB), collapsed when the victims were constructing its ceiling portion at the Jogappa layout in Amruthalli," a police official said.The victims are reportedly from Bihar and West Bengal.Though about 20 labourers were rescued from the mishap spot, at least 10 more are feared trapped under the debris of the 40-feet deep circular tank meant to treat about 110 million litres of sewage water per day."The incident occurred around noon when about 50 labourers were working with hollow blocks and cement on centering inside the tank for the sewage treatment plant, spread over a 30-acre plot," said the official.Fire and emergency service brigades rushed to the spot after the police and the contractor alerted them about the incident.A team of National Disaster Relief Force has been deployed to search and rescue the trapped workers from the crash spot. "Though the labourers were wearing helmets, they did not have protective body suits, added the official.The police filed a case against the contractor and the Board for failing to take precautionary and protective measures.