Singh, who was serving life imprisonment in the Presidency Jail for kidnapping Khadim owner Partha Pratim Roy in 2001, was killed by fellow inmate Nizam in the morning after being repeatedly hit by a brick, ADG prison Adhir Sharma said.
Sharma said an injured Happy Singh was taken to the jail hospital were he was administered five stitches and was later on sifted to M R Bangur Hospital for better treatment.
"The reason behind Nizam attacking Happy is still not known, but he is not mentally stable. Earlier on one occasion, Nizam had chopped off another inmate's ear. We are interrogating him," Sharma added.