Over the course of two years, a research team scoured archives and galleries for documented depictions of Palladio — some with labels saying they portrayed the architect — and whittled those down to about a dozen works. Culture ministry experts then took X-rays, infrared images and stratigraphic sections of these portraits to date them and — in some cases — determine whether the labels identifying the sitter as the architect had been added at a later date.

The examinations were fruitful. It took that team “around 15 minutes,” for instance, to determine that a portrait that had been widely believed to be a contemporary 16th-century depiction of Palladio was actually a 19th-century work, Mr. Magani said.

The Italian State Forensic Department, a national police agency, then examined the works using forensic techniques typically used in “cold cases,” with the extra challenge of working on ancient portraits, not photographs, said Vittorio Rizzi, a police official.

They compared facial features to see whether the portraits depicted the same person (most didn’t), and they used age-progression techniques typically used to identify fugitives from the law to see whether the British portrait of the young Palladio would look plausible when the subject was aged (it didn’t).

In the end, two seemingly legitimate portraits emerged. One came from a private collection in Moscow, and once belonged to Ivan Zholtovsky, a dominant figure in 20th-century Soviet and Russian architecture. “Once we examined this work, we determined, with no reservations, that it was a real and authentic 16th-century painting,” and the original prototype of other paintings, Mr. Magani said.

The other was a small, unknown painting depicting Palladio wearing a hat that was bought in a New Jersey antiques shop and was originally part of a larger series of portraits of notable persons painted in the 16th century.