OPP say officers were initially responding to reports of a domestic incident involving a firearm in Trent Lakes.

Ontario's police watchdog, the Special Investigations Unit, says it's investigating after an Ontario Provincial Police officer and a civilian were shot and killed in connection with an incident on Manitoulin Island on Thursday.

Investigators say the crash involved two commercial trucks, and resulted in a chemical spill on the Key River Bridge.

Police say the officer suffered non-life threatening injuries in the incident early Tuesday morning on Highway 400 and was released from hospital later that day.

Police said the officer was struck just before 1 a.m. on the highway near Finch Avenue.