MADRID — Until recently, the Prado’s copy of the Mona Lisa — one of dozens made over the centuries — was not much of a draw.

Then, Ana Gonzalez Mozo took an interest.

Over the past two years, Gonzalez, a researcher in the museum’s technical documentation department, has used all manner of modern-day techniques — X-rays, infrared reflectography and high-resolution digital images, among others — to make, and then document, an unlikely finding.

It turns out that the Prado’s Mona Lisa is not just any 500-year-old copy. It was most likely painted by someone who was sitting right next to Leonardo da Vinci, trying to duplicate his every brush stroke, as he produced his famous lady with the enigmatic smile.

When Leonardo adjusted the size of the Mona Lisa’s head or corrected her hands or slimmed her bosom or lowered her bodice, so did whoever was painting the Prado’s Mona Lisa.

“It had to be painted at the same time,” Gonzalez said. “Someone who copies doesn’t make corrections because they haven’t ever seen the changes. They can see only the surface of the painting.”

The discovery is primarily important for what it reveals about the real Mona Lisa, a painting that has been darkened by layers of aging lacquer.

The copy, now restored, offers details that are obscured in the original Mona Lisa. For instance, the copy shows an armrest where none can be seen in the original, and reflectographs show a much clearer image of her waistline.

“What is really important about the copy is that we can see how Leonardo worked,” Gonzalez said. “We know something new about his creative process.”

The copy, which also shows a much younger-looking figure, has once again ignited a debate about whether Leonardo’s Mona Lisa should be restored as well. Gonzalez says this is a hard call for the Louvre because people are so used to the way the painting looks now. But she cannot help being curious.

Most of the time, Gonzalez spends her hours looking beneath the surface of the Prado’s masterpieces, searching for insights into the artists’ methods and thinking. And there, she said, she has found great treasures. Many important paintings have sketches or first tries — adjusted and reworked — under the final image. Sometimes, she said, the work underneath is even more fascinating than the painting itself.

“I get to see what only the artist saw,” she said. “And he saw it five centuries ago.”

Some art magazines have speculated that the Prado’s Mona Lisa was painted by Leonardo’s lover. But Gonzalez has no patience for such gossipy talk. “That is irrelevant,” she said. “We don’t know that. And that is not what the work here is about.”

Until two years ago, the Prado, which inherited the painting with the rest of the royal collection in 1819, displayed it but never suspected its significance. It was catalogued without fanfare as an anonymous copy, painted on poplar.

The copy’s background was black, and the painting was covered in a layer of dark varnish, which gave it a yellowish glow and further diminished its vibrancy.

But the Louvre was planning a special exhibition of Leonardo’s work and, because it did not want to move the original Mona Lisa from its protected area, wanted to borrow the Prado painting as a stand-in. A casual comment by one of the Louvre curators, asking whether the painting had ever been studied, got Gonzalez thinking.

The next day she took her infrared camera into the gallery and got to work. Just the first pictures were enough for her to conclude that the two paintings had been produced in tandem. After that, it was just a question of watching the evidence pile up.

Perhaps the most exciting discovery was that the painting’s original background had been obscured by a layer of black paint, a practice sometimes used in the 18th century. Luckily, a layer of lacquer protected what was under it. So, once the paint was removed, the same Tuscan background as in Leonardo’s painting appeared, offering a tantalizing preview of what might be seen if Leonardo’s Mona Lisa were restored.

There is no doubt, however, that the Prado painting was not a copy made by Leonardo himself. While the corrections are identical, the lines are not. “Like I write an A and you write an A, you can tell it is not the same,” Gonzalez said.

Parts of the Prado copy are beautiful, she said, like the hands. But in general, it is not nearly so fine a painting.

Just why it was made remains an open question. It could have been simply for a pupil’s instruction or a double commission.

Gonzalez seems somewhat indifferent to the attention her recent discovery is getting. She said she had participated in far more spectacular discoveries. For instance, she said, X-rays and infrared reflectographs show that Tintoretto sketched nude figures under his clothed ones.

But, somehow, it is the copy of the Mona Lisa that everyone is talking about.

“It has grabbed people’s imagination,” she said. “She is an icon.”

The Prado’s Mona Lisa is on loan to the Louvre until June.