INDIANAPOLIS (AP) — Indiana State Police are hoping a photo of a knife blade used to kill one of four young Indianapolis restaurant workers slain in 1978 could lead to a break in the unsolved 40-year-old case.

The workers were abducted from a Burger Chef restaurant in Speedway on Nov. 17, 1978, as it closed.

Twenty-year-old Jayne Friedt , 18-year-old Ruth Shelton , 16-year-old Mark Flemmonds and 16-year-old Daniel Davis were found dead two days later in rural Johnson County, south of Indianapolis.

Friedt, the restaurant’s assistant manager, was stabbed to death.

First Sergeant Bill Dalton released a photo Wednesday of a 4-inch-long knife blade removed from her body. The knife’s handle was never found.

Dalton says the killer was likely carrying the knife in a sheath, and someone may know who was wearing that weapon.