"Chatu Das (33) alias Sameer Das, an associate of Gopal Tewari, was arrested from Nalhati of Birbhum district this morning," Joint Commissioner of Police (Crime) Pallab Kanti Ghosh told PTI.
Police are also looking for Tewari, an alleged Trinamool Congress activist currently absconding, and recovered arms and ammunition from his residence.
Ghosh said, "Das led us to Gopal Tewari's house on Pathuriaghata Street from where we have seized two 7 mm pistols, one 9 mm pistol, one single-shot arm, five magazines, 77 rounds of cartridge for 9 mm pistol and 23 rounds of .303 cartridge along with some explosive substances."
Earlier, six persons were arrested in connection with the incident.
Sub-Inspector of Girish Park Police Station Jagannath Mondal was injured after being hit by a bullet during a clash between two parties at Singhi Bagan soon after the April 18 voting for the Kolkata Municipal Corporation.