The mystery of a missing wartime Enigma encoding machine began to unravel yesterday Tuesday when a parcel containing the stolen device was sent to the BBC.

The machine, one of only three in the world, was taken six months ago from a museum at Bletchley Park, near London.

Threats were made that the machine, which is about the size of a typewriter, would be destroyed if a ransom was not paid.

Bletchley Park was the nerve centre of British decoding efforts during the Second World War.

The secret work carried out there is thought to have helped shorten the war by two years. Enigma was the name given to the German military coding system used to direct ships, submarines and armies on all fronts of the Third Reich's battle to dominate Europe.

From the newsroom of the BBC World Service