Warwickshire police has confirmed that £113,000 went missing after being held in a "secure storage area" at its former headquarters.
The money was seized from criminals in 2009 and was being kept in a police building in the village of Leek Wootton when it disappeared last September.
The force, suspecting it may have been taken by a police officer, set up a secret investigation in an attempt to catch the culprits. But after a six-month inquiry no one has been caught and the force has decided to go public with an appeal for information.
The money had been seized in 2009 under the Proceeds of Crime Act, and was linked to a trio jailed for a variety of offences including possession of firearms in January 2010.
Initially the force had reported the case to the Independent Police Complaints Commission (IPCC) but they ruled the investigation should be carried out locally.
It is being led by the anti-corruption unit of Warwickshire Police's professional standards department.
A police spokesman said: "Warwickshire Police is conducting a criminal investigation into the theft of GBP113,000 from a secure storage area at its Leek Wootton premises.
"The investigation is being carried out by the Anti Corruption Unit of Warwickshire Police's Professional Standards Department.
"Warwickshire Police has formally referred this matter to the Independent Police Complaints Commission (IPCC), which has decided that it should be investigated locally. Warwickshire Police Authority has also been informed."
The Leek Wootton estate, which was mentioned in the Domesday Book, covers 55 acres of sprawling Warwickshire countryside.
The main house, known as Woodcote House, was built in 1861 as a family country home and was used as a home for injured US servicemen in World War Two, before being purchased by Warwickshire County Council in 1947.
It became the force's headquarters two years later and remained so until February last year.
Police, courts and criminal justice agencies in the area have since moved to a "one-stop" justice centre in Leamington Spa.
The force refused to confirm whether the money had been in a safe or in an evidence room when it was stolen.
Earlier this year the Daily Telegraph revealed that one crime takes place inside a police station every hour.
Figures obtained under the Freedom of Information Act showed more than 8,000 crimes take place inside police stations in England Wales each year.
Police cars, uniforms and even handcuffs have all been stolen from under the noses of officers.
Lancashire and Gwent each had a police car stolen in the past year. Surrey police has had four police cars stolen since 2000.
The Metropolitan Police, Britain's biggest force, had more than 2,500 crimes recorded at its police stations in each of the past three years. It has had five police cars stolen since 2008.
In Northamptonshire a man was stopped in the street after being caught wearing a police uniform he had stolen from a station.
In Derbyshire the force reported incidents of criminals stealing handcuffs, police radios, laptops and even a knuckleduster from stations in the county.
The same force also reported, to itself, the theft of police hats, stolen from the heads of officers in the street.