The province's Special Investigations Unit (SIU) has been called in after an incident in Petrolia Monday night.
Lambton OPP responded to a home in the town at about 7:30 p.m. for a report of a man in mental distress.
Once on scene, officers were alerted to the rear of the property and secured the area.
Around 10:45 p.m., the Emergency Response Team entered the rear of the property and discovered a 40-year-old man suffering from what's believed to be a self-inflicted gunshot wound.
Police said he was taken to hospital in critical condition.
The SIU is asking anyone with information or video evidence related to the incident to contact them at 1-800-787-8529.
The SIU is a civilian law enforcement agency that investigates incidents involving police officers where there has been death, serious injury, or allegations of sexual assault.
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