Twenty-six years ago, a group of recreational divers made a gruesome discovery in the Otonabee River in Peterborough, Ont.

On July 10, 1988, they found the severed head of an adult male that had been wedged between a rock and a log.

Dr. Kathy Gruspier

Dr. Kathy Gruspier, a forensic anthropologist, says that the man whose severed head was found in the Otonabee River on July 10, 1988, was between 40 and 74 years old at the time of his death. (CBC)

Police never managed to identify the deceased. And they also never found the rest of his remains.

What they knew for sure was that the unknown man was the victim of a homicide.

"His head has been removed from his body…with the use of a sharp tool and it's not a boat propeller or anything in the post-mortem environment," said Dr. Kathy Gruspier, a forensic anthropologist, in an interview with CBC News.

"It's obviously a knife, or something similar to that, that has been used to cut the head through the neck off the body."

Police are now hoping that the release of a new image on Thursday may help jog someone’s memory and thus help investigators find out who this person was.

'River man' mystery

Police say the sketches above are a 'two-dimensional approximation' of what an unidentified man looked like at the time that he was killed more than two decades ago. The slain man has never been identified. His severed head was found in the Otonabee River in Peterborough, Ont., on July 10, 1988. (CBC)

It's a two-dimensional approximation that shows what the man may have looked like at the time of his death.

The unknown man is believed to have been between 40 and 74 years old.

"With only a skull, we can't get a very narrow age range," said Gruspier.

The deceased was likely a smoker and may have been bald, or partially bald, at the time that he was killed. He had also lost most of his molars prior to death.

If the man can be identified, then police will be able to begin the process of identifying the person or persons responsible for ending his life.

Anyone with information about the case is asked to contact the Peterborough Lakefield Community Police Service at 705-876-1122, ext. 555, or to call Crime Stoppers at 1-800-222-8477.

Click on the video above to see a full report from the CBC's Sue Sgambati.