For an artist with such a distinctive style, it’s surprisingly difficult to authenticate works by Leonardo da Vinci. Even "Salvator Mundi," sold in 2017 for a world-record $450 million as a genuine da Vinci, has many critics who say it lacks Leonardo's flair and refuse to accept its attribution. So when highly-respected researchers decide a piece was likely created by the master’s hand, it’s a big deal. That’s the case with "Monna Vanna," a drawing of a nude woman that experts from the Louvre now cautiously say may have been made by Leonardo.

Agence France-Presse reports that the charcoal drawing has been in the collection of the Condé Museum, north of Paris, since 1862. The Duc d’Aumale, the son of Louis-Philippe, France’s final monarch, bought the piece for the collection, writes Elian Peltier at The New York Times. At that time, the work was attributed to Leonardo.

Peltier reports it wasn’t until the 20th century that Renaissance experts studying the work decided it was created by someone in Leonardo’s studio. To celebrate the 500th anniversary of the artist’s death, researchers decided to take a closer look at the "Monna Vanna" to see if they could definitely attribute the work.

AFP reports that a previous study established that the work was created during da Vinci’s lifetime, and the paper it is drawn on, Peltier notes, was produced in the area between Florence and Venice. The image resembles da Vinci’s famous "Mona Lisa," and the hands are in almost the exact same position. Curators believe the charcoal sketch was likely made in preparation for producing an oil painting of the nude subject.

In the new study, undertaken at the world-renowned Center for Research and Restoration of the Museums of France (C2RMF) beneath the Louvre, scientists examined the image of a semi-nude woman more closely. Microscopic examination reveals that the portrait was drawn from the upper left-hand portion of the page toward the bottom right, which indicates the primary artist was likely left-handed, like Leonardo. Though some cross hatching at the top of the drawing appears to have been done by a right-handed person, throughout the rest of the artwork “left-handed charcoal marks pretty much everywhere,” said Mathieu Deldicque, curator of the Condé Museum, in a statement.

Finding those marks is a breakthrough. Peltier reports that experts originally doubted the work was from Leonardo’s hand because of those right-handed marks. Before the recent analysis, they could not discern if the creator of the rest of the work was right or left-handed.

The left-hand evidence and the style of the work, including techniques used like Leonardo's "sfumato," in which transitions between colors are softened, don’t seal the deal, but are pushing curators closer to attributing at least some of the work to Leonardo. “There is a very strong possibility that Leonardo did most of the drawing,” Deldicque told AFP. “It is a work of very great quality done by a great artist.”

There is circumstantial evidence that Leonardo had a part in the drawing as well. AFP reports that it’s possible the sketch was made in preparation for a full oil painting of the nude figure. “So many students of Leonardo have painted naked Mona Lisas or written about it, that we are almost certain that Leonardo painted one,” Deldicque told The New York Times.

In fact, reports Matthew Robinson at CNN, in June "Monna Vanna" will return to the Condé Museum to be the star attraction in a show commemorating Leonardo’s death that will bring together some of those other “naked Mona Lisas.”

So what would convince curators beyond a shadow of a doubt that the drawing is an authentic Leonardo? Deldicque told Peltier curators may never be absolutely certain about the piece, which makes it just as mysterious as the "Mona Lisa" itself.