Saidie May loan record.

The lore of the landscape was as irresistible to its owner as its beautiful brush strokes: Renoir had painted it, Baltimore collector Saidie May said, for his mistress on a linen napkin at a Paris restaurant along the Seine.

So how did the small painting wind up in a $7 box of junk at a West Virginia flea market more than eight decades after May’s ex-husband, Herbert L. May, purchased “On the Shore of the Seine” from a Paris gallery in 1926?

The mystery, which generated headlines this month when an Alexandria auction house announced that it would sell what it believes is that Renoir, became clearer this week when a Washington Post reporter entered the library at the Baltimore Museum of Art. In a box full of Saidie May’s letters and artwork receipts lay one major clue: records showing that she had lent the painting to the museum in 1937. The discovery startled museum officials, who had already said the flea-market Renoir never entered their institution.

But armed with the loan registration number, museum officials dug up in their collection records an even-more-astounding clue about the Renoir’s journey. An old museum loan registration document revealed that the tiny landscape, measuring 51 / 2 by 9 inches, was stolen Nov. 17, 1951, from the BMA — shortly after May’s death.

Now the painting’s highly anticipated auction by the Potomack Company has been canceled. The FBI is investigating, and museum officials are trying to learn more about the painting’s theft. They couldn’t explain why it does not appear on a worldwide registry of stolen and lost art.

“Obviously, we take our responsibility for our collections and the things entrusted to us very seriously. We have to do more research and get to the bottom of the real story, and we’re still in the midst of that process,” BMA Director Doreen Bolger said. “We have a lot of written and printed records, and they are filed in many areas of the museum.”

The new details could trigger a legal showdown over the painting’s ownership among several players: the historic Baltimore museum; the company that insured the painting and paid a $2,500 claim for the stolen artwork; the six-year-old auction house; and the Virginia woman who unwittingly purchased the Renoir at the Harpers Ferry Flea Market.

Bolger was clear about where the painting belongs: in the BMA’s May Collection. “We want the painting back. That painting was associated with her, and she’s one of the most important donors in the museum,” she said. “It was her decision that it would come to us.”

But it’s not as clear-cut to Elizabeth Wainstein, the Potomack Company’s president. Because Herbert L. May is listed as the buyer by the French gallery where the piece was first sold, she’s not certain that Saidie May technically owned it, said Wainstein, who wants more proof that the Renoir was pilfered.

“There is a strong indication, but we want to see a police report,” she said, adding that the landscape known as “Paysage Bords de Seine” will remain at her auction house until the matter is settled. “We would not sell the painting until we know the proper owner. We just need to the know the truth.”

The true owner of the painting might be the company that insured the Renoir at the time of its disappearance, said Christopher A. Marinello, executive director and general counsel of the London-based Art Loss Register, the world’s largest private database of stolen and lost art. In the mid-20th century, most art insurers had policies stipulating that they are entitled to stolen artwork that is recovered and for which they’ve paid claims, he said.

“Does the insurance company own the painting? Of course they do,” Marinello said. “When an insurance company [back then] paid out on losses, the title resided with the insurer.”

Museum officials aren’t sure who insured the painting. “We don’t know with complete certainty,” Bolger said. “I don’t know what the rules of insurance were at the time. Remember, this was 1951. I was only 2 years old at the time.”

Many of the museum’s records, Bolger said, are not as organized as they could be. When news of the flea-market Renoir surfaced in early September, Bolger said museum officials checked permanent-collection records to see if they had ever had it. They found no records of the landscape, which is dated to 1879.

They figured that was the only place they needed to check because May, who has a wing at the BMA named for her, had bequeathed her art collection to the museum. But the museum did not check its loan records.

It wouldn’t be the first time a May artwork has been stolen. The museum informed May in 1946 that it lost a French illuminated manuscript from its Renaissance room, along with a small leather-bound book with a fleur-de-lis on the cover.

“I feel that both items were deliberate thefts, and if two people work together one can draw the guard’s attention while the other vaults the rail in that room,” a museum official wrote May in a letter stored in the museum’s library.

On Friday, Baltimore police provided a copy of the original police report, from 1951. James M. Porter Jr., executive assistant with the museum, told an officer that sometime between 4 p.m. Nov. 16 and 1 p.m. Nov. 17, “some one stole” the painting. “There was no evidence of forced entrance,” the report states.

Wainstein, Potomack’s president, said the Virginia woman who made the flea-market find was disappointed. But the woman also immediately agreed to halt the sale until the FBI determines the rightful ownership of the painting, which the auction house estimates is worth $75,000 to $100,000.

Sharon Flescher, an art historian and executive director of the nonprofit International Foundation for Art Research, said the value of an artwork depends on its size, subject, condition, quality and rarity. One tiny Renoir landscape sold for $35,000 in June, she noted, while larger, more high-profile works can command millions.

The Virginia woman, who wants to remain anonymous, bought the landscape in 2010 for $7 in a box with a doll and a plastic cow. She stashed the box for nearly two years before her mother suggested that the painting might be a real Renoir.

The woman brought the painting to the auction house in July. Potomack checked with the Art Loss Register to make sure the piece hadn’t been stolen and confirmed its authenticity with the Paris-based Bernheim-Jeune gallery, which sold the painting to the May family in 1926 and keeps a registry detailing the ownership histories of Renoir pieces.

Susan Helen Adler, a great-niece of Saidie May who has written a book about her, said the news about the Renoir saddened her.

“I hope it gets returned to the museum where it belongs and where it was originally given to them for a purpose,” Adler said. “It would be wrong for someone else to buy it and know that it had been stolen.”

Lynda Robinson and Magda Jean-Louis contributed to this report.