The deceased was identified as Nayek Amir Hossain, 55, commander of the Ansar post (security camp) and a resident of Panchhari upazila in the district.
Ansar member M Hashem said he had an altercation with Rafiqul Islam at the Ansar camp over the duty allotment around 3pm. At one stage, Rafiqul took his rife to shoot Hashem.
Amir stepped in to pacify Rafiqul. However, Rafiqul squeezed the trigger of the rifle as soon as the post commander held the gun.
Bullet pierced through his chest, leaving him dead on the spot, Hashem said.
Rafiqul went into hiding with his riffle and 84 bullets after the incident.
Officer-in-charge of Dighinala Police Station Shahadat Hossain Titu said police recovered the body and sent it to hospital for autopsy.
Police and army members launched drives to arrest the killer, the OC added.