(CNN) -- Peter Hohenstatt was skeptical at first, especially when he learned the drawing dated to about 1500.

The sketch was "absolutely Leonardesque," the University of Parma art historian thought, but it was probably the product of one of the master's students, imitators or admirers. When a technical exam showed the drawing originated closer to 1473, his skepticism waned.

The reason? Leonardo da Vinci was an apprentice until the late 1470s. He didn't have any students, imitators or admirers of his own yet.

"I can't be sure it's a Leonardo drawing, not scientifically or any other way," said Hohenstatt, "but I'm highly convinced that we have here one of the first drawings. I'm quite convinced it's one of his first portrait sketches."

Hohenstatt isn't alone. Da Vinci expert Luigi Capasso told CNN there is a "very high possibility that this sketch is by Leonardo da Vinci."

The object of their fascination is titled simply, "red pencil drawing of a profile of a man's head looking to the left." It was found about 70 years ago tucked into a book -- and like many objects of artistic intrigue, it has a long and twisting story, regardless of whether it's the product of the Renaissance master.

The two men's convictions are based on artistic similarities to other da Vinci works as well as the makeup of the sketch's paper, which they say is similar to the paper da Vinci used in other sketches.

The final word, though, must come from the Biblioteca Ambrosiana, the library in Milan, Italy, that houses the Codex Atlanticus, the largest collection of da Vinci's works.

Hohenstatt conducted the historical and artistic analysis of the drawing. Capasso, an anthropology professor at Gabriele d'Annunzio University in the medieval town of Chieti, Italy, handled the scientific and technical side of the work.

Capasso, who also serves as director of the university's museum, was credited in 2005 with reconstructing a da Vinci fingerprint from his manuscripts and the painting "Lady with an Ermine." The fingerprint is one of the only biological traces of da Vinci, Capasso said.

Though the men cite several reasons for their conclusions, two elements are the most convincing, said Capasso.

First, the composition of the 5½-by-3½-inch sheet of paper is almost identical to the paper used in three da Vinci sketches housed at the Uffizi Gallery in Florence, including "The Landscape No. 8P," which da Vinci dated himself: August 5, 1473. All four works show evidence of lead salt treatment, which da Vinci commonly used to whiten his paper before applying inks and pigments, Capasso said.

The second element is where things get interesting: In volume 12, page 1033 of the Codex Atlanticus, there is a blank space with a glue silhouette that "corresponds impressively" with the paper on which the red-chalk sketch is drawn, Capasso said.

The glue on page 1033 even has a small imperfection that matches an imperfection in the glue on the back of the drawing.

Watch some of Capasso's and Hohenstatt's findings

Capasso said he has the chemical composition of the glue but cannot conduct a comparison with the codex until Biblioteca Ambrosiana permits an analysis.

Ambrosiana's vice prefect, Pier Francesco Fumagalli, declined to discuss the timing of the test. But in a letter to the sketch's owners, Fumagalli wrote that the library would permit a non-invasive analysis of the glue on page 1033.

The test will have to wait until at least 2012, he wrote, because the codex is in the process of being preserved.

The discovery

Cristina Gerbino, the daughter of one of the sketch's current owners, has been tasked with publicizing the discovery. She said the drawing was originally found in a book belonging to Cardinal Placido Maria Tadini, the onetime archbishop of Genoa. When Tadini died in 1847, a notary from the Ponzellini family bought the cardinal's Montcalvo home and all its belongings, Gerbino said.

In 1940, the last Ponzellini descendant moved to Genoa and, while reorganizing the books, noticed the cover of an old tome coming apart. Upon inspection, "a small, red charcoal drawing appeared, a portrait of a man in profile, which had obviously been intentionally hidden," Gerbino said.

The descendant framed the drawing and began gathering the cardinal's documents to research its origin, but World War II interrupted his endeavor. He died in the early 1950s, leaving the framed sketch hanging on a wall in his study -- but not before he conveyed the story behind the sketch to his nephew, Giuseppe Boero.

Fast-forward about five decades.

Amedeo Barile, who restores antiques, was doing work at Boero's home and saw the drawing still preserved under glass. He "decided to buy it and continue the research in order to discover the identity of the artist behind the drawing," Gerbino said.

Barile, along with his colleague and friend, Francesco Gerbino, took ownership of the drawing and in 2001 commissioned studies by Consulting Scientific Group Palladio in Vicenza.

The basics came first: Examining the drawing's surface with infrared rays showed that it had been executed in one sitting, rather than in intervals, according to an analysis. And a scanning electron microscope determined the paper was made from rags, typical of paper produced before the 18th century.

Neither was a huge revelation until Barile showed the sketch to a scholar and author who suggested it might be the work of the great Leonardo di ser Piero da Vinci himself.

Hoping to find evidence of the sketch's origin, Barile and Hohenstatt, who had been studying the drawing from a historical perspective, traveled together to Biblioteca Ambrosiana and examined a photocopy of the Codex Atlanticus. On page 1033, they found a collection of riddles and prophecies, for which da Vinci had a predilection, alongside two glue outlines: one large, one small.

On page 1035, they found the drawing that matched the larger outline, but there was no drawing in the codex matching the smaller outline.

Barile and Hohenstatt returned to the library later with tracing paper containing the glue outline from the back of the red-chalk sketch. They held it up to the photocopy of the codex, turned it upside down and were delighted.

"The match was perfect," said Cristina Gerbino, who in December set up the website Leonardo Rediscovered to plug the finding.

Unraveling the mystery

Hohenstatt, who has written books on da Vinci and his fellow Italian master Raphael, said the characteristics that initially led him to believe the sketch was influenced by da Vinci are the same ones that now convince him it is his actual work.

The sketch contains evidence that whoever drew it used both hands. Though da Vinci is generally regarded as a southpaw, many drawings from his first Florentine period "suggest an execution with both hands," Hohenstatt wrote.

Then, under the chalk near the neckline of the man in the drawing is a "slightly sketched head of an animal," perhaps a dog or a bear, Hohenstatt said. It's well documented that da Vinci studied both animals during his early Florentine period, which ended about 1482.

The drafting of "one sketch over the other, it's very common in his drawings," Hohenstatt said, explaining that artists of that era often recycled paper because it was so expensive.

A comparison of the drawing to other da Vinci portraits reveals certain similarities: the elaborateness of the eyes, nose, cheek and chin among them. The subject of the portrait also communicates "an intelligence" evident in at least five other da Vinci works, Hohenstatt said, including da Vinci's rendering of Isabella d'Este, who was known as the first lady of the Renaissance.

In the hair of the sketched subject, Hohenstatt also noticed a technique of creating light and shadow that da Vinci used up until about 1500.

Hohenstatt conceded that the red chalk, or sanguina, employed in the sketch was not commonly used by da Vinci until his first Milanese period, which began in 1482. But at least one other red-pencil da Vinci drawing has been attributed to the early Florentine period, and "The Landscape No. 8P," which da Vinci dated 1473, has evident traces of red pencil on it, the professor said.

As of about 1500, da Vinci was rarely doing portraits in profile, Hohenstatt said, but one of the master's documents mentions that he brought portraits with him to Milan, "so portraits were an interest he had when he was in Florence."

"If it's a Leonardo da Vinci drawing, this type of drawing has to be early, before the 1490s," Hohenstatt said, adding that all these elements -- the ambidexterity, the details in the face and hair, the obscured animal head, the red chalk traces -- "are marginal features, but all these things together make this a typical Leonardo drawing."

Again, though, these elements on their own weren't enough to convince Hohenstatt. If not for Capasso's scientific analysis, Hohenstatt would still have his doubts.

To compare the paper on which the sketch is drawn to other da Vinci sketches, Capasso used a colleague's analysis of the paper used for the three sketches at the Uffizi Gallery.

"The chemical composition of the papers is, more or less, about the same, in the same epoch and in the same geographic area," Capasso said, adding that there are traces of heavy elements that are so rare they help provide a sort of signature for the paper -- essentially, where it was made and used.

"In fact, contrary to the common elements in all the papers (which do not distinguish one kind of paper from another), traces of heavy elements are so rare and so characteristic as to be specific of one kind of paper," the professor wrote in his analysis.

The analysis showed that "The Landscape No. 8P" had traces of lead, copper, zinc, strontium and manganese in the chemical composition of the paper, the same as the red-chalk sketch. Another sketch, a study for "The Battle of Anghiari," had all the elements except zinc. A third sketch, a study for "The Adoration of the Magi," had all the elements except strontium and manganese.

Losing a da Vinci

At least two experts believe the sketch is a da Vinci, and Biblioteca Ambrosiana is intrigued enough that it plans to permit testing of a national treasure. If that's the case, exactly how does a priceless artwork from an Italian master become lost in the first place?

Turns out it's not that rare an occurrence.

In 2005, an Italian art detective said he believed that the da Vinci fresco "The Battle of Anghiari" was hidden behind a Giorgio Vasari mural in the Palazzo Vecchio in Florence. The detective is still working to unearth the artwork.

In 2008, the Louvre in Paris announced it had found three da Vinci sketches on the back of his oil painting "The Virgin and Child with Saint Anne." The following year, an Italian journalist said he found an early pencil self-portrait in da Vinci's Codex on the Flight of the Birds.

There have also been discoveries that pitted da Vinci authorities against each other. In 2009, experts claimed that a portrait purchased at auction in 1998 for $19,000 was actually da Vinci's "La Bella Principessa." Others disagreed. Also that year, a medieval historian said he found what he believed to be a self-portrait while studying a picture collection in Acerenza, but the historian quickly found critics.

"We have tremendous loss of drawings, sketches and manuscripts of Leonardo da Vinci," Hohenstatt explained.

Parts of the codices were simply dismantled, he said. Other drawings were cut out and assembled in different books.

Da Vinci's assistant and favorite student, Francesco Melzi, administered da Vinci's estate and upon his own death left the artworks to his son, Orazio.

"(Orazio) didn't have any more interest in the cultural understanding of the drawings" and gave away some of the pieces to his friends, Hohenstatt said.

When Orazio died, his heirs sold the remaining collection. Sculptor Pompeo Leoni, who originally assembled the Codex Atlanticus, took many of the drawings to Spain in the 16th century and split up many original manuscripts. One of them was lost until 1966, when it was rediscovered in the National Library in Madrid.

Many works were eventually donated to Biblioteca Ambrosiana in the mid-1600s, where they remained until Napoleon Bonaparte plundered Milan in 1796, Hohenstatt said. Several drawings and manuscripts remain at the French National Library and the Institute of France.

So, the professor explained, losing a da Vinci wouldn't have been too difficult because his works have been rather well traveled since his death in 1519.

Finding one, on the other hand -- and proving it's the genuine article -- is a markedly more difficult process.