The SIU has been called in after an incident involving a member of the public and a police officer at the Pen Centre Mall.
One week ago, on St.Patrick's Day the NRP say a uniform officer was at the St.Catharines Mall responding to a disturbance.
On Monday March 21st, police say they became aware that a member of the public may have suffered injuries during an interaction with a police officer involved in the investigation of the disturbance.
The NRP say they found out through social media.
Police say they contacted the Special Investigation Unit (SIU) and advised them of the information.
The SIU has now commenced an investigation of the matter.
In compliance with legislation the NRP is prohibited from releasing further information pending the conclusion of the SIU investigation.