Three people are dead and a fourth is clinging to life following a crash involving six vehicles on County Road 22 in Lakeshore.
OPP say the collision happened just after 6 a.m. Wednesday, forcing the closure of all lanes between Manning Road and West Pike Creek Road.
Investigators say the drivers of two vehicles were pronounced dead at the scene, while a passenger was rushed to hospital and later pronounced dead.
Provincial police say a fourth person has now been taken to hospital in London via air ambulance.
Others involved in the crash are also being assessed for injuries.
County Road 22 was closed between Manning Road and West Pike Road as investigators examined the scene. It has since reopened.
The preliminary investigation has determined a vehicle entered the opposite direction of traffic and struck a vehicle which caused minor damage but then the wrong way vehicle struck a second vehicle head on.
The head on collision caused the involved drivers to be pronounced dead at the scene, while a passenger was rushed to hospital and pronounced dead.
The investigation remains ongoing, and police say further information will be provided as it becomes available.
Police are also reminding motorists not to call emergency lines for information about road closures, noting those calls can tie up resources needed to respond to emergencies.
Correction
8:50 a.m. - An earlier version of this story reported that three people had died in the collision, based on information provided by Ontario Provincial Police. Police have since issued updated information, stating that a passenger who was transported to hospital regained vital signs after arriving at hospital. OPP now say two people, not three, have died as a result of the crash. The story has been updated to reflect the new information.
Correction
9:04 a.m. - This story has been updated several times as Ontario Provincial Police released new information. Police initially reported that three people had died in the collision. OPP later said one passenger who had been pronounced deceased regained vital signs after arriving at hospital, reducing the death toll to two. In a third update, police said that passenger was subsequently pronounced deceased. The story has been updated to reflect the latest information from investigators.