The identity of a severed head found in a skip in Mepal, Cambridgeshire may never be known.
Police have formally closed the investigation into the discovery made by a quarry worker in May 2016.
Forensic experts believe the head was that of a man between the age of 30 and 50. They say he may have died as long ago as the 1960s, but most likely in the last 10 years.
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.
A forensic dentist and specialists from The Natural History Museum were also involved in the investigation.
Police believe the man may have been hit by a train in Bedfordshire in "a tragic accident".
"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."