Three Dutch Old Master paintings stolen 21 years ago in a daring Christmas Eve heist from a San Francisco museum have been returned anonymously to a New York auction house, officials said Thursday.

The 17th-century paintings, one of them attributed to Rembrandt, were left in a box at the William Doyle Galleries on Nov. 2 and were quickly recognized as works taken from San Francisco's M.H. de Young Memorial Museum in 1978.

"We have no idea who did it," Carolyn Macmillan, a spokesperson for the Fine Arts Museums of San Francisco, said Thursday. "The mystery hasn't been solved, but at least the paintings have been returned. We are grateful that they will be back in their proper place."

The most famous of the paintings is "Portrait of a Rabbi," which art specialists originally believed to be a Rembrandt but which some experts now think may have been painted by one of his students or a skilled copyist.

The other recovered works include Aert van der Neer's "River Scene at Night" and "Interior of the Church of Saint Lawrence, Rotterdam" by Anthonie de Lorme.

The whereabouts of a fourth painting stolen in 1978, Willem van de Velde's "Harbor Scene," is not known.

Auction house officials said an unidentified man dropped off the three paintings during a weekly open house for walk-in appraisals. No one got a good look at the man, who vanished before the package was noticed.

Alerted by an anonymous telephone call, auction house employees called the police to open the box, which they feared might contain a bomb. Instead, they found the three paintings, which Alan Fausel, the galleries' director of paintings and a former San Francisco museum employee, recognized as belonging to the de Young.

"We are proud to have had a role in the recovery and return of the paintings to the de Young," Fausel said in a statement.

Museum officials tempered their euphoria over the return of the works by noting that all three had been seriously damaged, including "Portrait of a Rabbi," which someone had apparently attempted to clean.

That painting, if it were in perfect condition and proven to be an actual Rembrandt, could be worth as much as $20 million, but museum officials said Thursday they suspected that, given its condition and dubious authenticity, it could be worth less than the $1 million estimated in 1978.

The other three stolen paintings were estimated to be worth a total of $75,000 in 1978.