Lambton OPP say a swimmer was transported to hospital in critical condition after being rescued from Lake Huron.
Emergency crews responded to Pinery Provincial Park shortly after 5 p.m. Saturday for an unresponsive swimmer.
Police said the individual had been swimming in Lake Huron when they went into distress.
Good Samaritans helped bring the swimmer to shore and immediately began administering CPR until first responders arrived and took over care.
A 50-year-old from Hannon, Ontario, is still in hospital, and the investigation is continuing.
The OPP are thanking members of the public who provided assistance prior to the arrival of emergency responders.
Police said conditions on Lake Huron can change quickly.
Always wear a lifejacket or personal flotation device (PFD) when swimming in open water, as lifejackets save lives.
Anyone with information related to this incident is asked to contact Lambton OPP at 1-888-310-1122.
