LOS ANGELES — The great mystery of who stole the Willem de Kooning painting “Woman-Ochre” by cutting the canvas out of its frame at the University of Arizona Museum of Art in 1985 has still not been solved.

But the issue of where the painting has been — and why it has been out of public view since its 2017 discovery by a New Mexico antiques dealer, has become clearer. It turns out that the work, said to be worth at least $100 million, has been in museum storage for months, awaiting approval from the FBI to send it for restoration.

“The FBI has kept this an open case,” Olivia Miller, the Arizona museum’s curator of exhibitions, explained. “The painting has been at the museum but it was still considered evidence, so it wasn’t allowed to leave until this past November.”

Now, the museum, in Tucson, is staging a coming-out-of-storage cocktail party on Sunday, with the first public viewing of the painting since 1985, before sending it to the Getty Center in Los Angeles in April for conservation. The Getty plans to exhibit the painting during the summer of 2020, after treatment, before returning it to the Arizona institution.

The challenge is bringing the 1955 canvas — an example from de Kooning’s celebrated but also contested “Woman” series, known for their grotesque, even savage renderings of the female nude — back to near-original condition. Ulrich Birkmaier, the Getty Museum’s senior paintings conservator, and Tom Learner, the Getty Conservation Institute’s head of science, have teamed up for the project. They expect the process to take at least a year.

“Unfortunately, the painting suffered considerable damage as a result of the theft,” Birkmaier said. “In 1985 when the painting was cut out of its frame — actually a very clean cut — the thief ripped it off the lining, which caused a lot of horizontal cracks.”

He believes the cracking was exacerbated by another action the thief possibly took: rolling up the canvas tight enough to tuck it under a coat.

Birkmaier described paint “lifting off” from the cracked areas, which will need to be secured, and noted that the original canvas border wrapped around the stretcher was left behind during the theft. “The treatment will most likely involve a relining of the canvas during which it can be reunited” with its border, he said.

“Once the painting is consolidated, cleaned and restretched, there will be endless debates with my team and Arizona about retouching,” Learner said, adding, “My instinct is that we are going to err toward maintaining the existing condition because that’s where conservation is at the moment.”

The Getty team also plans to analyze the artist’s choice of materials. In her book on de Kooning, Susan Lake, a conservator, showed that starting in the 1940s he worked with glossy house paints as well as artists’ oils. It appears he might have used both in “Woman-Ochre.”

The Getty Museum exhibition will feature the painting and the restoration process. In 2012, the Getty struck a similar agreement with the University of Iowa for Jackson Pollock’s 1943 painting “Mural.”

Miller, the Arizona curator, said the Getty beat out other teams because of its “cutting-edge science” and commitment to educating the public. “We were looking at the work they did in collaboration with University of Iowa on the Jackson Pollock painting, which produced this incredible research, as a model,” she noted.

Miller vividly remembers receiving the phone call in August 2017 from David Van Auker, an antiques store owner in Silver City, New Mexico. He had bought “Woman-Ochre” from the estate of Jerry and RitaAlter, not recognizing it as a de Kooning, let alone as stolen property, until he put it on display and some customers took immediate interest. “He was super calm,” Miller said. “He introduced himself and said: I bought some items from an estate and I have your stolen de Kooning.”

She did not disclose the insurance value for the painting today, and it’s hard to say how much its damage would dampen a price or its dramatic back story would aid it. An earlier “Woman” painting brought $137.5 million in a private sale.

As for the unveiling Sunday, Miller was reluctant to reveal details: “The painting will not be left alone, that’s for sure. We’re hiring extra security.”

This article originally appeared in The New York Times.