Chandigarh, July 18 : Four persons were killed due to suspected gas poisoning at a biogas plant in Haryana's Kurukshetra district on Wednesday, police said. The victims included a man and his two sons, who were choked to death at Dera Rampur village. The bodies were sent for post-mortem to the Civil Hospital in Kurukshetra, around 110 km from here. Police said that the exact cause of death was being probed.