Bit by bit, Leonardo’s lost masterpiece revealed itself: the Mona Lisa mouth, the subtle brushwork, the gossamer glaze.

Now, several years after the world learned that a painting long believed to be a copy was in fact the work of Leonardo da Vinci, the $127.5 million masterpiece, “Salvator Mundi,” has landed in the middle of one of the most astonishing art scandals in decades.

What began in the rarefied realms of Monaco and Geneva, with allegations of stolen Picassos and marked-up Modiglianis, has now jumped the Atlantic and drawn the attention of the U.S. Justice Department.

Federal prosecutors, following the lead of European authorities, have opened an inquiry into one of the art world’s consummate insiders, Yves Bouvier -- including his dealings with the rediscovered Leonardo, according to people familiar with the matter.

The move marks the first time that federal authorities have trained their sights on a scandal that has shaken Europe’s notoriously private ecosystem of art dealers, middlemen and collectors. While still in its infancy, the U.S. probe also underscores prosecutors’ general concerns about the opacity of the market in art -- which, like high-end real estate, can serve as a conduit for money laundering.

Forty Works

For months Bouvier, 52, has been battling his one-time patron, the billionaire Dmitry Rybolovlev. The Russian oligarch, by his own count, has spent more than $2 billion on close to 40 works purchased through Bouvier over the past decade.

Bouvier swindled Rybolovlev out of many millions by sharply marking up prices on several acquisitions and pocketing the difference, the Russian billionaire alleged in a complaint to Monaco authorities. One work in question is Modigliani’s “Reclining Nude With Blue Cushion,” which Rybolovlev bought for $118 million from hedge fund legend Stephen A. Cohen. Rybolovlev later discovered during a lunch in St. Barts that Cohen had sold the piece for $93.5 million.

Justice Department prosecutors are now examining various art deals Bouvier struck on behalf of clients, including transactions involving not only the Modigliani but also works by Klimt and Rothko, focusing on the extent to which he may have misrepresented to clients how much he’d marked up prices, people familiar with the matter say. Of those works, the Leonardo is perhaps the most famous. If the investigation advances, Bouvier could face fraud charges in the U.S.

Bouvier, who operates out of the Geneva Freeport, a vast, tax-free storehouse for artwork and other valuables, has said he has done nothing wrong and charges what the market will bear.

“We have not been contacted by the U.S. authorities and are unaware whether -- or how -- any such inquiry has been initiated,” said Daniel Levy, an attorney for Bouvier at McKool Smith in New York. “During the course of this commercial dispute over paintings, the other party has repeatedly attempted to use law enforcement to further his own private objectives.”

A spokesman for Rybolovlev didn’t immediately comment. A Justice Department spokesman declined to comment.

Monaco Penthouse

Until now, this drama has unfolded mostly in Monaco, where Rybolovlev, 49, lives in one of the world’s most expensive neighborhoods, in a penthouse that overlooks Monte Carlo’s yacht-filled marina.

It was in Monaco last February that police arrested Bouvier as he entered the lobby of Rybolovlev’s Belle Epoque residence. Bouvier denied the allegations in the Russian billionaire’s complaint that Bouvier misled Rybolovlev about the prices of works he was buying. In November, an appeals court rejected Bouvier’s request to have the criminal charges of fraud and complicity in money laundering against him dropped, and Rybolovlev said in a statement that he was “pleased” that the case was going forward.

One of the masterpieces Bouvier procured for Rybolovlev was none other than “Salvator Mundi,” a 16th-century painting of Christ. With its authorship unclear, the painting was acquired by a consortium of art dealers that included Robert Simon, a specialist in Old Masters. Simon then brought the painting in 2005 to Dianne Modestini, a renowned restorer and research professor at New York University in Manhattan.

Yellowing Varnish

Modestini recalls placing the oil painting on an easel in her Upper East Side studio. It was covered with yellowing varnish and overpainted from previous restoration attempts. Modestini began painstakingly restoring the piece. It wasn’t until years later that she knew for sure that she had restored one of the rarest pieces of art in the world.

Experts now agree Leonardo painted the piece for Louis XII of France, probably around the year 1500. Following the discovery, the painting was exhibited in London’s National Gallery.

Then, in 2013, a mystery buyer acquired “Salvator Mundi.” It was Rybolovlev, according to later news reports. He purchased the painting, via Bouvier, though a trust. According to the Monaco complaint, the billionaire paid $127.5 million for the work, which was roughly $50 million more that he later said the seller had received, alleging Bouvier pocketed the difference.

However the dispute plays out, Modestini laments that her lost Leonardo has disappeared from public view -- a common complaint as famous works of art are acquired by private collectors.

“When you own a painting such as this, you have a responsibility to hold it as part of the public trust," Modestini said, adding the public should be “able to experience the power of paintings like the Leonardo.”