Last summer, Christie’s sold the last known Rembrandt portraits owned by private collectors. But, a mysterious unsigned portrait in the style of the Dutch master recently popped up in an attic in Camden, Maine—and sold for far more than its estimate of $10,000–$15,000 at Thomaston Place Auction Galleries. Although the details surrounding this work remain murky, an anonymous private collector from the U.K. scooped it for $1.4 million.

“I never imagined I’d help close a deal for over a million dollars,” says Thomaston Place Auction Galleries’ staff member Zebulon Casperson, who represented the winning bidder over the phone, in a statement. “It feels like a shared victory.”

The new portrait turned up during a standard house call that Kaja Veilleux, the founder, appraiser, and auctioneer behind Thomaston Place Auction Galleries, made to an private estate in Camden.

Veilleux concluding the sale of Portrait of Girl, after Rembrandt. Courtesy of Thomaston Place Auction Galleries.

“We often go in blind,” Veilleux remarked in the auction house’s release. “The home was filled with wonderful pieces, but it was in the attic, among stacks of art, that we found this remarkable portrait.”

The perfectly preserved, purportedly antiquated work of oil on cradled oak panel depicts a teenage girl dressed in an austere black robe, with a frilly white collar and white cap, against a plain background. A gold, hand-carved Dutch frame rings the scene.

While the artwork’s style, particularly its striking use of light, indeed evokes Rembrandt, the real revelation is on the verso—which features a slip from the Philadelphia Museum of Art recording that Mr. Cary W. Bok, a late descendant of Philadelphia’s Curtis Publishing Company fortune, loaned it this artwork in 1970. The slip attributes the painting to Rembrandt, titling it Portrait of Girl. Although the slip also noted that Bok was based in Camden, it’s impossible to say whether his family put this portrait up for sale. Veilleux has pledged to keep the consignor anonymous.

The label affixed to the back of the portrait. Courtesy of Thomaston Place Auction Galleries.

The Philadelphia Museum of Art was unable to clarify which show the institution may have loaned this portrait for, but a representative did note that such a slip does not equal an authentication. Thus, Portrait of Girl hit the block as a painting done “after Rembrandt” on August 24. According to Live Auctioneers, the work is from the 1630s, an era where Rembrandt was overseeing portrait commissions as the head of art dealer Hendrick Uylenburgh’s studio.

But, lingering uncertainty hardly stymied the excitement. Seven hours into the second day of the Summer Grandeur sale in Thomaston that Saturday, bidding on the portrait opened at $32,500, already well above estimate, and ballooned rapidly over 60 successive offers by 11 bidders—two in person, and nine over the phone—in increments of $25,000. As bidding reached $900,000, the pack whittled down to three, and then two, phone bidders.

According to the auction house, the painting now represents one of the most expensive artworks ever sold in Maine. By comparison, Portrait of Marten Looten (1632) remains the most expensive authentic Rembrandt ever sold, after hammering for $33.8 million in 2015.