Khulna Metropolitan Police’s Detective Branch and Sadar police arrested Shibli Hossain Rubel, 46, in the city’s Nirala residential area.
Police said the operation took place on Wednesday night.
Police said they recovered a foreign-made pistol, a magazine and one round of ammunition during the operation.
They said the items were found in a shopping bag in the suspect’s bedroom.
Deputy Commissioner of Police Mohammad Ahaduzzaman Mia said the arrest followed an investigation into the shooting of Abdul Haque Hawlader at Dolkhola Motleber Mor on August 4.
Mia said investigators analysed CCTV footage and used technology to identify Rubel.
Police alleged that Rubel is a member of the Palash and Ashik groups.
Police also alleged that he has multiple cases involving theft and robbery.
Police said an Arms Act case was being processed.
The precise timing of the police briefing on Thursday remains unclear, with accounts placing it in the morning and at noon.