Sub-Inspector Palthurai, who was arrested in connection with Thoothukudi custodial deaths, succumbed to COVID-19 on Sunday night.
A few weeks ago Palthurai was admitted to the jail hospital after he tested positive for COVID-19.
Earlier on July 28, head constable Murugan and constable Muthuraj, two accused in the case had also tested positive for COVID-19.
The Madurai bench of the Madras high court had on July 28 directed the Central Bureau of Investigation (CBI) to file a status report in a sealed cover on August 17 in the Thoothukudi custodial deaths case.
As many as 10 accused policemen, arrested in connection with the custodial deaths of Jayaraj and his son Bennix in Tuticorin, were lodged at the Madurai Central Prison.
P Jeyraraj, 55, and his son J Benicks, 31, were allegedly tortured in the police station leading to their deaths.
The father-son duo was arrested on June 19 and was lodged in Kovilpatti sub-jail for keeping their mobile shop open during a curfew imposed amid the COVID-19 lockdown.