Thieves have stolen more than 200 items - with an estimated value of $5m (£2.67m) - from Russia's prestigious Hermitage art museum in St Petersburg.
The Hermitage has one of the finest art collections in the world
The thefts are thought to have been carried out over a number of years - police believe as many as 30 years.
Jewellery and precious enamels were amongst items stolen from the museum.
The museum said staff had been involved in the theft. The curator in charge of most of the items died suddenly at work when the inventory was being checked.
The Hermitage, one of the world's most famous art museums, has more than two-and-a-half million works of art, housed in more than 1,000 rooms.
"There are many strange aspects of this affair, but unfortunately, there is no doubt that it did not happen without the participation of museum staff," a museum statement said.
It said the affair had exposed "serious moral problems" among staff, who had "neglected their duties and responsibilities".
The items had not been on show in the public galleries, but had been kept in storage.
The museum's curator, Mikhail Piotrovsky, said: "Items kept in storage are not insured."
Many of the rooms in the museum have poor ventilation and security, with staff often opening windows to let in fresh air, the AP news agency said.
"The Hermitage theft unfortunately is not the first such occurrence when objects or documents disappear from archives or museums representing our country's birth," Boris Boryaskov, head of the Rosokhrankultura cultural protection agency, told the RIA-Novosti news agency.
The Hermitage collection includes world-famous masterpieces of Impressionist and Flemish art, and was started by Russian empress Catherine the Great in 1764.
The museum statement said the affair showed the acute need for fundamental reform of the management and culture in Russia's museums, which were struggling to survive.