PIACENZA, Italy (AP) — Art experts have confirmed that a painting discovered hidden inside an Italian art gallery’s walls last month is Gustav Klimt’s Portrait of a Lady, which was stolen from the gallery nearly 23 years ago.

The authentication of the painting announced Friday solved one of the art world’s enduring mysteries – where did the missing work end up? – but left several questions unanswered, including who had taken it and whether it ever left the museum’s property.

A gardener at the Ricci Oddi Modern Art Gallery in the northern city of Piacenza who was clearing away ivy noticed a small panel door on a wall outside and opened it. Inside the space, he found a plastic bag containing a painting that appeared to be the missing masterpiece.

“It’s with no small emotion that I can tell you the work is authentic,” Piacenza Prosecutor Ornella Chicca told reporters Friday while two police officers stood on either side of an easel bearing the recovered painting.

Portrait of a Lady depicts a young woman sensually glancing over her shoulder against a dreamy moss green background. Klimt finished the painting in 1917, the year before he died. The Ricci Oddi gallery acquired it in 1925 and reported it missing in February 1997.

Since the gardener’s discovery on Dec. 10, the canvas had been kept in a vault of a local branch of Italy’s central bank while experts used infrared radiation and other non-invasive techniques to determine if it was the original Portrait of a Lady.

Experts said the painting was in remarkably good condition. One of the few signs of damage was a scratch near the edge of the canvas that may have resulted “from a clumsy effort to remove the portrait from its frame,” said Anna Selleri, an art restorer from the National Gallery in Bologna.

The experts who did the verification work found persuasive evidence in the work of their peers more than two decades ago.

An Italian high school student, preparing for her graduation exams in 1996, noticed striking similarities between the painting that would go missing a year later and an earlier Klimt work of a woman with a similar posture and gaze but wearing a hat and scarf, accessories that the artist didn’t include in Portrait of a Lady.

Intrigued by the observations of the student — who went on to become an art researcher herself – experts back then examined the artwork in the Piacenza gallery’s collection and found that Klimt had painted it on top of an earlier portrait of a woman.

Those studying the work in recent weeks, with the aid of X-rays, saw the earlier portrait. Selleri said the radiation analysis revealed that while painting the later portrait, Klimt didn’t redo much of the face, but used whitish pigment from the earlier version for the skin.

Portrait of a Lady was officially listed as missing on Feb. 22, 1997, but might have been snatched from a gallery wall a few days earlier, during the exhibit preparation work.

So who stole the painting? Chicca said police were studying some traces of organic material on the recovered canvas in hopes they might provide leads.

Asked if authorities knew whether the piece had ever left the gallery’s grounds, investigators said that’s something else they hope to find out.

As for why and when the painting ended up stashed behind a wall, journalist Anne-Marie O’Connor, the author of a book about the dramatic fortunes of Klimt’s Portrait of Adele Bloch-Bauer, has an educated theory.

Before the 1990s, Klimt was largely “considered an Austrian painter, but his stature really grew at this point,” O’Connor said. When Portrait of a Lady was taken, the value of the art nouveau artist’s paintings was “soaring,” she said.

O’Connor ventured that perhaps whoever took the painting stowed it behind the gallery’s walls while waiting for news about the heist to die down but the stolen work proved “too hot to handle.”

“It would have been hard to sell it to a private buyer” on the so-called gray market, O’Connor noted in a phone interview from London.

Some of Klimt’s works have experienced stunning turns of fortune.

O’Connor’s 2012 book The Lady In Gold chronicled the ultimately successful effort by a woman to gain back Klimt’s Portrait of Adele Bloch-Bauer.

That portrait was snatched from the Bloch-Bauer home in Vienna in 1941 by a Nazi officer. The woman, Bloch-Bauer’s niece, later sold the painting to cosmetics mogul Ronald Lauder in 2006 for $135 million.

Another celebrity Klimt piece was a second portrait of the woman, Portrait of Adele Bloch-Bauer II. Oprah Winfrey eventually purchased that painting and reportedly sold it a few years ago for $150 million.

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D’Emilio reported from Rome.