An oil painting by French impressionist Pierre-Auguste Renoir that was stolen from a private residence in Tokyo in 2000 was auctioned in February at Sotheby’s in London for about £1.05 million ($1.6 million), investigative sources said Monday.

The work, “Madame Valtat,” painted in 1903, is a portrait of Suzanne Valtat, wife of Renoir’s close friend and painter Louis Valtat.

The male owner of the home in Setagaya Ward discovered the theft of the Renoir and five other pieces, including those by Russian-born French painter Marc Chagall and Japanese artist Ikuo Hirayama, in August 2000, the sources said.

The painting was auctioned off on Feb. 5 in London and the owner, who noticed that the piece had been sold at auction, informed police in Japan in March.

Japanese investigators have not yet identified the thief. The Renoir was not listed on an international database of lost and stolen artworks and thus slipped through a check at Sotheby’s for stolen items, allowing the auctioneer to put it under the hammer, the sources said.

The owner intends to seek the return of the work, but it may be difficult to specify the person who purchased it at the auction because Sotheby’s keeps client information confidential.

Sotheby’s told Kyodo News that the seller of the Renoir had acquired it legitimately in 2000 and provided representations and warranties on rightful ownership of the painting. This means the painting was probably sold to the seller shortly after it was stolen.

The auctioneer said it has been looking into the possibility the work was a stolen item and is in discussion over the issue with the parties involved.

It is believed the owner notified police in Japan at the time of the theft, but neither he nor the authorities contacted the stolen items database to have the works listed.