In the latest twist to one of Italy’s most bizarre cases of stolen art, the widow of a former gallery director is being investigated as police try to piece together the mysterious circumstances surrounding the theft and recent rediscovery of a Gustav Klimt painting.

Prosecutors in Piacenza are questioning Rossella Tiadina, the widow of Stefano Fugazza, who was managing the Ricci Oddi modern art gallery when Klimt’s Portrait of a Lady disappeared from it in February 1997. Tiadina, whose home was searched, is being investigated on the premise of handling stolen goods.

The painting was found concealed in a wall of Ricci Oddi in December by two gardeners as they cleared ivy. Experts confirmed its authenticity in January.

The latest turn in the investigation was prompted by an entry written by Fugazza, who died in 2009, in his diary, in which he confessed that he had contemplated organising a fake theft of the painting to raise publicity for an upcoming Klimt exhibition.

“I wondered what could be done to give the exhibition some notoriety, to ensure an audience success like never before,” he wrote. “And the idea that came to me was to organise, from the inside, a theft of the Klimt, just before the show (exactly, my God, what happened), for the work to then be rediscovered after the show began.”

Fugazza later wrote: “But now The Lady has gone for good, and damned be the day I even thought of such a foolish and childish thing.”

The extract was first published in an article by the BBC in December 2016, and then in another by Ermanno Mariani, a journalist for the local Piacenza newspaper Libertà, who also cited the diary insertion two years later in his book about the case.

“The case is such a mystery,” he told the Guardian. “I don’t believe the widow has anything to do with it, but it might be that, if Fugazza was involved, she could be implicated for having been in the same house as a stolen object.”

An investigation into two men who confessed to stealing, and returning, the painting is ongoing. The men, believed to be part of a gang involved in dozens of thefts in Piacenza, had originally declared themselves the perpetrators in a letter to Mariani in 2016.

The motive for their recent confession is thought to be aimed at getting more lenient prison terms for another theft. The confessions were made more than 20 years after the Klimt painting was stolen, meaning they would avoid prison for that theft as the statute of limitations for the crime had expired.

Portrait of a Lady, valued at €60m, was was painted between 1916 and 1917 by the Austrian art nouveau master. The painting went missing during preparations for an exhibition in Piacenza aimed at showing it off, after the revelation that it was the only “double” Klimt known to the art world.

The discovery was made by an art student who realised it had been painted over another Klimt piece previously believed lost: Portrait of a Young Woman, which had not been seen since 1912.