It is not a city famous for olive groves, romantic piazzas or baroque fountains, but Buffalo, in gritty upstate New York, does have one feature that even Florence or Rome might be jealous of: a "Pietà" – a painting of the Virgin Mary cradling the body of Jesus – by Michelangelo.

The people of Buffalo could be forgiven for being unaware of this until now. Rather than hanging on a museum wall, the unfinished painting was for years stuffed behind a sofa in the house of the Kober family. Even the Kobers, who took the picture off their wall after it was hit by an errant tennis ball, weren't sure whether it was priceless or worthless. If it is a Michelangelo, it is considerably closer to the former than the latter, and, if similar works at New York'sMetropolitan Museum of Art are any guide, could be worth anything up to $300m (£190m).

The quest to find out the truth was started, on his father's urging, by Lieutenant Colonel Martin Kober in 2003 after his retirement from the US Air Force. He took his job seriously, vaguely conscious that the bundle behind the couch might one day become the most important art find of the century. He contacted auction houses, art historians and even visited a leading expert on Michelangelo in Italy, Antonio Forcellino.

For Mr Forcellino the visit by Lt. Col Kober marked the beginning of an unexpected adventure. He was skeptical at first that the 19 by 25-inch painting on wood which the Kobers had half-jokingly christened the "Mike" could be the real thing. Mr Forcellino's research – outlined in his new book, La Pietà Perduta (The Lost Pieta) – points to the picture being by the master. Michelangelo's 1499 sculpture masterpiece, Pietà, is housed at St Peter's Basilica in the Vatican.

"I'm absolutely convinced that is a Michelangelo painting," Mr Forcellino told the New York Post. "In reality, this painting was even more beautiful than the versions hanging in Rome and Florence. I had visions of telling them that there was this crazy guy in America telling everyone he had a Michelangelo."

Part of what helped to convince him was the result of X-ray and infrared examinations of the painting which reveal multiple alterations, not something you would expect in a copy of an original, as well as an unfinished section near the Madonna's right knee. By tracking the provenance of the work he was also able to work out how it ended up in Buffalo.

Mr Forcellino believes the work was painted by Michelangelo, in about 1545, as a gift for a friend. It then passed to two Catholic cardinals before ending up in the hands of a German baroness. She bequeathed it to a favourite lady-in-waiting who was the sister-in-law of Mr Kober's great grandfather.

Definitive proof that this work was by the brush of Michelangelo may be hard to come by, but if sufficient numbers of experts agree with Mr Forcellino that should be enough to make the Kobers very wealthy indeed.

Buffalo residents (and the rest of us) will meanwhile have to remain patient. The picture is not on public display in the Kober home or anywhere else. They have it stashed away, not behind the family furniture, but in the vault of a bank.

LOST & FOUND

Portrait of a Lady as Flora - Giambattista Tiepolo

Tiepolo's work, painted in the mid-18th century, was found in the attic of a French chateau, where it had been placed because its original owners thought it was too risqué. After being discovered in 2008, it went on auction at Christie's, where it was expected to sell for between £700,000 and £900,000, but ended up fetching more than £2.8m.

Tres Personajes - Rufino Tamayo

When Elizabeth Gibson, who was on her way to buy a cup of coffee, spotted a large painting lying among rubbish bins in Manhattan in 2003, she almost left it before deciding to take it home with her. Hanging it on the wall of her flat, she spent a number of years trying to find out more about the picture, before finally discovering that Tamayo's abstract masterpiece (painted in 1970) had been stolen in 1987. It was returned to its owner, who then decided to sell it at auction, and it ended up going for more than $1m.

Portrait of Effie Gray - John Everett Millais