NCB officers said the existence of the lab indicated that Bongaon, which is located near the India-Bangladesh border, was no more a mere transit point for trafficking heroin produced in north India to Bangladesh.

“The quality of heroin arriving here from states like Rajasthan and Madhya Pradesh was becoming increasingly inferior and was losing demand in the international market. To increase its purity, kingpin Bimal Mondal had set up a crude lab at his farmhouse in Chandpara,” said K Shankar, the zonal director of NCB.

Acting on a tip-off, NCB officers, at first, arrested Mondal’s associates — Abhijit Mondal (31) and Abdul Majid Mullick — from Chandpara. They were carrying 11 kg of acetic anhydride, a chemical used for purifying heroin. Their interrogation led the NCB to the Chandpara farmhouse from where they seized heroin and another 27 kg of acetic anhydride. Bimal’s father Santosh Mondal (65), who was present at the farmhouse, was also arrested. The NCB team also raided Bimal’s residence in Bongaon.

“Mondal was in Kolkata when we were carrying out the raid. We have learnt that he is hiding in Durgapur. We are trying to track him,” said Shankar.

Preliminary investigation has revealed that 40-year-old Bimal runs a vehicle rental service. “Bimal Mondal is reasonably wealthy. He owns two houses and around 40 bighas of land,” said NCB officers. He got involved in drug smuggling around five years ago, the officers said.