A witness alerted police early Tuesday to a vehicle in the St. Marys River west of downtown, but emergency responders couldn't save the man inside, according to a Fort Wayne Police Department news release.
Officers responded about 3:35 a.m. to the 1600 block of West Jefferson Boulevard, where they found a vehicle in the river south of the bridge across from Swinney Park, police said.
The person inside – whom authorities didn't identify Tuesday – initially was determined to be in life-threatening condition, but later died, police said.
Investigators said they believe the vehicle was traveling west on Jefferson when it left the road for an unknown reason.
No other motorists were involved, police said.
Police closed the boulevard to traffic for several hours.
The man's identity and his cause and manner of death will be released following an autopsy by the Allen County coroner's office.
Anyone with information is asked to call police at 260-427-1222 or Crime Stoppers at 260-436-7867.