POINT EDWARD, Ont. -- Ontario's provincial police hope a new appeal to the public will help reinvigorate an investigation into a four-decade-old homicide.

Karen Caughlin, 14, of Sarnia, was dropped off early on March 16, 1974, in front of a friend's house on Brock St. South in the city.

Investigators say Caughlin never entered the home and a farmer discovered her battered body later that morning in a shallow ditch by a gravel sideroad northwest of Petrolia.

"So we have a blank in our timeline from 12:50 p.m. until Karen is discovered at 9:50 a.m.," OPP Det. Insp. Chris Avery said Thursday.

Police have pinpointed specific locations in the area where there may have been witnesses, including the Sarnia Arena and Sarnia Collegiate Institute and Technical School. They have also released a case chronology video.

"We're hoping that that may stimulate memories regarding observations made at the time," Avery said.

It's hard to predict how much people remember 40 years later, he said.

"Maybe they have a recollection of a specific event that may prompt them to bring themselves back to that era and the memories surrounding that," he said.

Police have said the autopsy revealed that Caughlin's injuries were consistent with having been struck by a vehicle.

However, circumstances such as her purse being located several days later in a ditch north of Petrolia, led investigators to rule her death a homicide.

Even a $50,000 reward for information leading to the conviction of her killer hasn't led police to an arrest in the case.