Police and forensic archaeologists are examining a human skull which was found in woodland in Buckinghamshire.
It was discovered on Sunday morning by a man walking his dog in Wendover Woods, Aylesbury Vale.
The area has been cordoned off and Thames Valley Police officers are searching the site.
Det Sgt Pete Foy said officers and other specialists were trying to determine the nature and origin of the bone, which could take several weeks.
He appealed to people who visit the woods regularly and, in particular, an area known as The Bowl to come forward if they have any information about the skull or have found other bones in the woods.