This Europe: Mystery of Mussolini, his mistress and the letters no one knew were missing until now

The missing letters are the stuff of a librarian's nightmare. Scholars of Italian history have waited more than half a century to get their hands on the correspondence Claretta Petacci sent to her lover, Benito Mussolini, between 1933 and 1945.

But this week, announcing, with the passage of exactly 70 years, the opening of the archive's first year, Maurizio Fallace, the superintendent of the state archive at Rome, had to confess that letters and diaries from one of the key years of their liaison, 1937 – the year of the Rome-Berlin-Tokyo Pact, the crucible of the Second World War – were missing.

Petacci, Mussolini's "air-headed and unlosable last mistress" according to one ungallant historian, was 29 years younger than Italy's Fascist dictator and stuck to him like a limpet; she was executed by his side in 1945, then in the final humiliation her body was hung upside down next to his in a Milan square.

Nobody knows exactly what is in the 600 letters and 15 volumes of diaries that Petacci wrote during the long and bumpy love affair, or if they know they are not telling.

When Petacci's sister Miriam returned to Italy from Spain in 1950 the letters had disappeared into the files of the Ministry of the Interior.

She went to law to get them back, but the Supreme Court ruled in 1956 that the state had a right to hang on to them because of their historical importance.

The claim, though scarcely credible, is that nobody has so much as taken a peek since.

Mr Fallace succeeded as superintendent last October and, preparing to open the first batch of documents for inspection, he discovered the loss. All 170 of the archive's employees were pressed into hunting them down, without success.

"There are 100km of documents here," Mr Fallace lamented; every inch of every stack was scoured. Then the police were called in.

The supposition now is that they have been stolen, though by whom and for what purpose is likely to remain a mystery.