One of the world's leading specialists on Leonardo da Vinci (search) cast doubt Saturday that fading frescoes in forgotten rooms of a Florentine convent might be the work of the Renaissance master or that of his pupils in the early 1500s.

With Dan Brown's smash-hit novel, "The Da Vinci Code" (search) sparking interest in the mysteries of the artist's life, the recent hypothesis by researchers in Florence that the rooms served as a studio for Leonardo has grabbed the imaginations of many.

If the artist and his pupils worked in the convent, might not they have done the frescoes, including one depicting birds — a motif that tickled Leonardo's fancy?

"They look absolutely standard 1480-ish bird studies," Martin Kemp (search), art history professor at the University of Oxford and a renowned Leonardo specialist, said in a telephone interview from Britain.

He said that although he hasn't seen the frescoes themselves, he has examined "high-quality" photos of them, and "they're obviously late 15th-century frescoes," done in the style of artists working a couple of decades before Leonardo was believed to have occupied the rooms, from 1500-1503.

Kemp's view largely echoed skepticism voiced earlier by other league art historians.

"I don't think it's a very convincing story yet, and there is no real evidence," James Beck, an art history professor at Columbia University in New York, said on Friday. "It's really in an early stage of research."

The rooms, which are closed to the public, are on the upper floors of the convent. Santissima Annunziata Monastery is now occupied partially by the Institute of Military Geography, some of whose researchers were intrigued by the rooms.

Those who think there's a good chance Leonardo might have painted in the rooms cite Giorgio Vasari's account that the convent let the artist stay there for free.

Skeptics point out that Vasari's lively biographies of Renaissance artists were written decades after the facts.

The birds in flight could have been done by Leonardo's school, Alessandro Vezzosi, director of a Leonardo museum near Florence, said Friday.

"The researchers made the hypothesis that these were the rooms where Leonardo and his pupils worked," said Vezzosi, who helped present the results of the research.

Vezzosi said Leonardo could have conceived or completed an early version of the Mona Lisa in the workshop, since the family of the probable subject of the painting, Lisa Gherardini, had links to the monastery.

For some reason, the frescoes on the rooms' walls hadn't been studied carefully.

"For the first time in this case, we see birds which are absolutely dynamic, animals which are absolutely vivid and remind us of the study done by Leonardo of birds in flight," said researcher Roberto Manescalchi, an architect at the geography institute.

But while Leonardo's fascination with flight is well documented, that's not enough to make the work on the walls his, skeptics said.

"There's nothing about these bird studies that are unique at that date," said Colin Eisler, a professor at New York University's Institute of Fine Arts.

Also in the rooms is an outline of a kneeling angel similar to Leonardo's Annunciation, which hangs in Florence's Uffizi gallery.

Kemp noted there was contemporary evidence of what Leonardo was working on in Florence in the years he was believed to be staying in the convent, including a couple of letters written by a priest who had been sent by a noblewoman, Isabella d'Este, to call on the artist.

The letters spoke about other projects keeping Leonardo occupied but nothing about the frescoes.

"Leonardo's name now is so electric," Beck said, referring to the frenzy over "The Da Vinci Code." But he cautioned that many tantalizing finds in the art history world soon fizzle.

Last fall, restoration specialists in the Umbrian town of Gubbio said they found a signature under years of grime on a religious banner which appeared to that of Raphael when he was in his mid-teens. But Beck said there already was skepticism in the art world over that hypothesis.