The Special Investigations Unit (SIU) said Niagara Regional Police officers were called to the area of Main Street West and Canal Bank Road around 9 a.m. for a dispute with neighbors.
When police arrived, a person of interest returned to his apartment and turned himself in.
The SIU said the Niagara Police Emergency Operations Unit (ETU) responded to the scene as well as a negotiator who attempted to speak with the man.
At around 2pm, five hours after the initial call, members of the ETU decided to force entry into the apartment.
Once inside, the SIU said one of the officers discharged his gun, striking the man. The agency did not say in its release Sunday afternoon what led to the shooting.
The man was taken to hospital, where he died.
The SIU, which is tasked with investigating police conduct that may have resulted in death, serious injury or allegations of sexual assault, has assigned three investigators and three coroners to the case.
The agency urges anyone with information to contact investigators at 1-800-787-8529.