In a shocking incident, a former army personnel allegedly killed his daughter-in-law and his tenant, his wife and one child with a sharp-edged weapon on Tuesday morning in the Rajendra Park area in old Gurugram.Another girl child of the tenant is critical and was admitted to a private hospital.Preliminary investigation suggests that extramarital affairs could be the reason behind the murder.The police came to know about the incident after the accused identified as Rao Rai Singh himself reached the Rajendra Park police station and informed them about the incident. Further investigation is on.During the probe, it also came to light that the quarrel between the accused and victim was going on for the last one and a half year."Four people including a child were killed with the help of sharp edged weapon. The accused is in police custody. An actual reason behind the killing would be known after a detailed investigation," Deepak Saharan, DCP (west) told media persons.