The identity of a severed head found in a skip may never be known after police formally closed the investigation.
The discovery was made by a quarry worker in Mepal on May 16, 2016.
Forensic experts identified the head as that of a man aged 30 to 50, who could have died as long ago as the 1960s.
Despite an extensive investigation it is still not known who he was, but police said there was no evidence to link him to any past criminal cases.
The discovery of the head led to a protracted search to identify who it might belong to, how they died and how the disembodied head came to end up in a skip.
Specialists from the Natural History Museum and a forensic odontologist (forensic dentist) also tried to identify the remains.
Police believe the man may have been hit by a train in Bedfordshire in “a tragic accident”.
Det Insp Jerry Waite of Cambridgeshire police said: “We believe the head may have been lodged somewhere along the structure of the bridge in Sharnbrook, and as part of the recent cleaning process has been dislodged, found its way into the skip and deposited at the quarry in Mepal.”
Detectives said the man might have died as long ago as the 1960s, but it was “more likely” his death occurred “about 10 years ago”.
With no leads, Cambridgeshire police confirmed the case had been “closed” but would reopen if more information came to light.
A spokeswoman for the force said: “We have utilised every opportunity available to us to try and identify this person but sadly at this stage their identity remains a mystery.”