The Special Investigations Unit has been called in after a 24-year-old man was found late Monday in Lake Ontario.
The SIU says Niagara region police got a call around 7 p.m. about a man "acting erratically" in Niagara-on-the-Lake.
Emergency crews also responded to assist the officers and a man was found in the water and attempts were made to resuscitate him.
The SIU says he was later pronounced dead in hospital.
The man's name has not been released.
The SIU investigates all incidents involving police that result in death, serious injury or allegations of sexual assault.
Note to readers: This is a corrected story; A previous version said the man's body was found.
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