Aman Singh, a sharp shooter, was shot dead in Jharkhand's Dhanbad jail on Sunday afternoon, police said.
"The incident happened around 3pm. Singh was shot dead in jail. A probe has started to find out exact cause of the incident and the men behind it," Dhanbad senior superintendent of police (SSP) Sanjeev Kumar told news agency PTI.
Singh, one of the accused in Dhanbad deputy mayor murder case, was reportedly shot during lunch time following a dispute among inmates, said another police official.
He was rushed to a hospital where doctors declared him brought dead.
The incident has raised concerns as Dhanbad deputy commissioner, SSP and other senior officials had conducted an inspection of Dhanbad jail on November 25 but did not find any weapons.
Singh was arrested by the Special Task Force (STF) of Uttar Pradesh near Mirzapur jail in May 2021.