The three men and a minor whom a trader in Entally has accused of extorting him are part of a larger extortion gang operating from a city jail, police said.

The three men were arrested and the minor detained on Wednesday for extorting and threatening the trader, the police said.

During investigations cops learnt that a person recently convicted of murder on the Subhas Sarobar premises in 2017 has been running the extortion racket from the jail where he is lodged, the police said.

Asish Sardar had been shot dead on the sarobar premises on June 29, 2017 when he had apparently refused to get intimate with a woman and his get his video clicked by some men.

Abhijit Chakraborty, 21, and Mohammad Wasim, 28, were arrested in connection with the murder. The trial was completed in January this year and Chakraborty was convicted of murder and sentenced to life imprisonment. Wasim was acquitted for want of evidence.

Chakraborty is now lodged in a jail in south Calcutta.

Investigators have found that he had been making extortion calls to people in the city, many of which have gone unreported, the police said. His friends and associates, including the minor, have been helping him run the racket.

Officers of the detective department’s anti-rowdy squad have found that Chakraborty had been making extortion calls from inside the jail while his accomplices have been threatening their target in person.

The police said they were thinking of tagging Chakraborty in the Entally trader extortion case.