Top code-breakers in Britain are unable to decipher a message found attached to the leg of a dead World War II pigeon.
The remains of the bird were found in a chimney in Surrey with a message attached.
Inside a canister on its leg was a thin piece of paper with the words "Pigeon Service" at the top and 27 handwritten blocks of code. This was given to the Government Communications Headquarters at the start of November this year.
Experts at the intelligence agency say it may be impossible to decode it without more information.
The BBC reports there had been speculation that the message might have been sent by an agent of the Special Operations Executive and that it was heading for Bletchley Park. But these theories have largely been discounted.
An undercover agent in occupied Europe would not use an official note pad in case he or she was caught with it in their possession.
An intelligence agency historian said the best guess is that the message was sent by a unit in the middle of an operation in Europe which was on the move and so unable to stop and set up the aerial for a traditional wireless message.
It also remains possible it could have been some kind of training exercise though - even perhaps for D-Day.
The BBC reports some 250,000 pigeons were used during the war by all services and each was given an identity number.
There are two pigeon identification numbers in the message - NURP.40.TW.194 and NURP.37.OK.76. It is unclear which one relates to the bird in the chimney.