The SIU has been called in after an incident at a St. Catharines variety store.
It happened around 5:30 Sunday evening, at the corner of Arthur Street and Northglen Avenue.
Police were called in and a taser used, an 18 year old man was rushed to hospital and is listed in serious condition.
A 77-year-old man and a 75-year-old woman were also taken to hospital with minor injuries.
Four investigators and two forensic investigators have been assigned to investigate this incident.
The Special Investigations Unit is asking any witnesses to contact the lead investigator at 1-800-787-8529.
The SIU is called into investigate cases when there has been a death, serious injury or allegations of sexual assault involving a police officer.