The controversial effort to find out whether a long-lost Leonardo da Vinci masterpiece lies beneath a fresco in Florence has been suspended without resolving a mystery that some have compared to a "Da Vinci Code" riddle.

The mystery surrounds a painting known as "The Battle of Anghiari," or "Fight for the Standard," which was commissioned by city officials for a meeting hall in the Palazzo Vecchio to commemorate a Florentine military victory in 1440. Contemporary accounts indicate that Leonardo began the wall painting in 1505 — but left it unfinished, due to problems he encountered with the experimental technique he was using to apply the paint.

Decades later, the city hall was enlarged and restructured, and in 1563 the Italian artist Giorgio Vasari painted a mural on one of the new walls. In the course of all that remodeling, Leonardo's painting disappeared. Today, it's known only from Leonardo's preparatory sketches and from copies inspired by the original.

Fast-forward to 1975: Maurizio Seracini, an Italian-born engineering professor and expert in art analysis at the University of California at San Diego, was back in his native Florence, studying Vasari's fresco. He noticed that a soldier in the fresco was waving a flag that read "Cerca Trova" (Seek and Ye Shall Find). Did this hint at the location of the lost Leonardo painting?

Over the years that followed, Seracini marshaled the expertise, technology and financial support needed to create a virtual reconstruction of the hall's layout before the remodeling took place. It looked as if there was a gap between the part of the wall where the "Cerca Trova" legend was painted and the older wall beneath. Armed with that information — plus funding from the National Geographic Society and backing from Florence's mayor, Matteo Renzi — Seracini won permission from Italian officials to drill six tiny holes into Vasari's wall and push camera-equipped endoscopic probes into the gap behind it.

The initial results were promising: Seracini said the team found "traces of pigments that appear to be those known to have been used exclusively by Leonardo." This March, National Geographic aired a documentary about the investigation, titled "Finding the Lost da Vinci." Heartened by the findings, Seracini asked for permission to conduct more sophisticated tests. The story was shaping up as a real-life "Da Vinci Code" thriller in the art world. (In fact, Seracini is mentioned in the Dan Brown novel as an art diagnostician who unveils "the unsettling truth" about a different work by Leonardo.)

Italian officials, however, were becoming increasingly unsettled about tampering with the 450-year-old Vasari mural. Some experts questioned whether there was really enough justification to go forward. "Vasari would never have covered a work by an artist he admired so much in the hope that one day someone would search and find it," Discovery News quoted Tomaso Montanari, an art historian at the University Federico II in Naples, as saying. "You would expect such a hypothesis from Dan Brown, certainly not from art historians."

In the end, cultural officials ruled that the scientists could drill one more hole for endoscopic tests, but couldn't do any further drilling after that. That meant the more sophisticated (and more intrusive) tests could not be conducted. Last month, Italian news outlets reported that the National Geographic Society was suspending the project "until further notice." 

Now Discovery News says that Florentine museum officials have given the go-ahead to fill in the six existing holes and take down the scaffolding that was used during the project. "This is how it ends," the Italian newspaper La Repubblica reported, "with strokes of stucco and paint, the search for Leonardo's mythical work."

More Leonardo da Vinci mysteries:

• Dig for Mona Lisa turns up a skeleton
• The anatomy that Leonardo couldn't copy
• Did Leonardo copy his famous 'Vitruvian Man'?
• Leonardo da Vinci ... fashion designer?
• Art experts hold mock 'Da Vinci trial

For more about the unsolved "Da Vinci Code" case, check out Rossella Lorenzi's report for Discovery News.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.