Koraput: Three persons, including two masons and a daily-wage labourer, died of suspected asphyxiation after entering a septic tank to clean it at Tarpaguda under Padmapur police limits in Rayagada district on Wednesday.The deceased were identified as Harishchandra Bir, Prakash Lima and Alok Karad all residents of Jibajagatpur village near Kenduguda under Padmapur police station limits, said police.According to sources, the three had been engaged to clean the septic tank of a house at Tarpaguda. One of them reportedly entered the tank first following which the others went inside one after another. They subsequently became unconscious, apparently due to poisonous gases or lack of oxygen inside the tank.On being informed, a fire services team from Padmapur rushed to the spot and launched a rescue operation. The three men were pulled out of the tank and taken out, but they were found dead.Police said the exact cause of death would be ascertained after postmortem. “Prima facie, the three appear to have died of asphyxiation after entering the septic tank. Further investigation is underway,” said IIC, Padmapur police station, B D Pradhan.“An investigation into the incident is underway. The bodies were sent for postmortem, and further action will be taken based on the medical report and the circumstances leading to the deaths,” said Pradhan.