A skull found amid the large-scale search for Corrie Mckeague was not that of the missing airman, police said.
It was found at a landfill site in Landbeach, Cambridgeshire at a time when police were trawling another landfill at nearby Milton for the missing 23-year-old.
Police said the skull was female and dated back to pre-1945. Mr Mckeague's family was informed of the find.
Mr Mckeague, of Dunfermline, was last seen in Bury St Edmunds in September.
A spokeswoman for Cambridgeshire Police said: "On April 14 a human skull was discovered at a landfill site in Ely Road, Landbeach, near Cambridge.
"Early indications of the age of the skull meant it was highly unlikely to be that of Corrie Mckeague, however Suffolk Police and Corrie's family were informed.
"It has since been established that the skull is female and dates back to before 1945.
"There are no suspicious circumstances therefore the investigation has been closed."
The spokeswoman said the skull was found by workers at the site and had been traced back to a house clearance of a man who "collected curios". The coroner was made aware of the discovery, she said.
On Friday, Suffolk Police confirmed it had ended its search of waste at the Milton landfill site.
Police also said on Friday an external force was reviewing the investigation.
On Monday, Suffolk Police said that until this review was completed the area of the landfill site searched would be left in "its current state" and would not be used for further waste disposal.
Corrie's mother Nicola Urquhart has urged the force to reconsider and is considering seeking an injunction to stop the site being backfilled.
More than 21,000 people have signed a petition calling on police to continue searching the waste site.
The RAF serviceman has not been seen since a night out in the Suffolk town when CCTV showed him entering a bin loading bay.
Suffolk Police said Mr Mckeague was known to "sleep in rubbish on a night out".
Det Supt Katie Elliott said the landfill search for Mr Mckeague had been "systematic, comprehensive and thorough".
Mr Mckeague's girlfriend April Oliver gave birth to their baby daughter Ellie in June.
On Facebook she wrote on Monday: "My little Ellie brings so much joy and happiness even at the hardest of times. Love you always."