ROME — It's an artistic mystery whose hottest clue is a fingerprint.

"The Adoration of the Christ Child" is attributed to Fra Bartolomeo, but a newly discovered fingerprint in the paint, along with stylistic similarities, are making experts think of Leonardo da Vinci, who sometimes left a digital imprint on his works as a sort of signature.

Near the completion of the painting's yearlong restoration, "a kind of yellowish halo could be seen in the sky in the upper left," the chief restorer, Elisabetta Zatti, said Tuesday, describing the fingerprint she found.

Attribution of the painting has long been in question, and some illustrious names have come up through the centuries — Raphael, Ghirlandaio, Lorenzo di Credi.

The key may lie in Krakow, Poland, where a Leonardo masterpiece, "Lady with an Ermine," bears the Renaissance master's fingerprint. Photos of the "Adoration" will be flown there next month for comparison.

Leonardo was big on code. Apart from fingerprints, he wrote backward in his notebooks, and used such symbolism as wild primrose, which represents resurrection, and the blue veronica flower, symbol of the eyes of the Virgin Mary. Primrose and veronica have shown up in the restored "Adoration."

The work, hanging in Rome's Galleria Borghese, is believed to have been painted in the late 15th century or early 16th, and depicts Joseph and Mary gazing down at the infant Jesus.

Perhaps the most striking revelation are Mary's large and somewhat masculine hands, a hallmark of many female figures in Leonardo's work.

"There are many details that make one think of Leonardo, like the stylistic power, the technique of 'sfumato,' the virile hands, the eyelids, and the expressive intensity of Saint Joseph, as well as that it's a work full of symbolic meaning," Zatti said in a telephone interview.

Leonardo pioneered the technique called sfumato which gives outlines a hazy edge and can lend both dreaminess and sense of heightened realism to a work. Many artists copied the technique, but Leonardo's use of it was unique.

Alessandro Vezzosi, the director of a museum dedicated to Leonardo near Florence who was not involved in the restoration, said the discovery was interesting, but cautioned that more research was needed.

"Fingerprints are very useful, and Leonardo's paintings and manuscripts are full of them," Vezzosi said. "If that is his fingerprint, it means at least that he has worked on that painting."

But Zatti said that if the fingerprint turns out to be Leonardo's, the painting could probably be attributed to him.

"It's difficult to imagine he would have left it on the painting of someone else," she said.