PORTLAND, Maine — The Mona Lisa attracts huge crowds of art lovers who wait in line for hours to glimpse a one-of-a-kind masterpiece. Except maybe it isn't.

A look-alike painting has art historians split over its origin.

While the true Mona Lisa hangs at the Louvre in Paris, its near twin is locked away from public view in a Swiss bank vault. NBC's Keir Simmons tracked down experts who had a chance to study them both, and he shared their very different findings in a report on the Today show on Tuesday, December 17.

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Jean-Pierre Isbouts is one of the scholars interviewed by Simmons. He says both paintings bear the hallmarks of Leonardo da Vinci's brushstrokes. In support of his case, Isbouts points out that the pigments in both are the same type used by Leonardo in the 16th century. He also cites manuscripts that refer to a copy of the Mona Lisa.

Martin Kemp applied his own artistic expertise and drew a different conclusion. He says the similarity between the two paintings argues against Leonardo's dual authorship. Kemp says Leonardo would made changes and improvements to his composition rather than copy himself so closely.

The Mona Look-alike first turned up in an English country house more than a hundred years ago. From there to the bank vault that serves as its current home, the painting passed through the hands of several owners. 

If it is proven to be authentic, the "other" Mona Lisa would be only the 16th officially recognized painting by Leonardo in the world. Five of the 15 acknowledged Leonardos are housed at the Louvre.

And just maybe, the Portland Museum of Art (PMA) is home to yet another. The painting of a woman with a familiar smile arrived at PMA in 1983 as a donation from Henry Reichhold.

It raised enough questions that the museum staff sent the painting to Harvard for analysis. Tests dated the painting's creation to some time before 1510, which puts it near the years of 1503 and 1507 when Leonardo painted the Mona Lisa.

Most experts agree that Maine's member of Team Mona Lisa is an early copy. But an alternate theory supposed it's actually a study painted by Leonardo himself in preparation for his magnum opus.

Interest in the Mainah Lisa took off as the "Da Vinci Code" became a cultural phenomenon. Published in 2003, the novel by Dan Brown imagined a centuries-old religious conspiracy that could only be unraveled by following the clues that Leonardo hid in his art. A movie adaptation came out in 2006 with Tom Hanks in the lead role.