A third suspect is being held over the theft of 221 items from the famous Hermitage art museum in St Petersburg, Russian media report.
The Hermitage has one of the finest art collections in the world
Last week, police detained two men over the heist. They were linked to one of the museum's curators, who died suddenly at work last year.
The theft - with an estimated value of $5m (£2.67m) - was discovered in July.
The Hermitage is home to a massive collection of sculpture, paintings and historic artefacts.
Police did not any reveal any details about the third suspect.
However, Russia's Interfax news agency - citing unnamed police officials - said he was the son of the curator who had been in charge of the collection where the theft occurred until her death last year.
Last week, police said they arrested two suspects after pursuing a lead from an antique dealer, who had returned one of the stolen items.
Police said the two men were suspected of stealing pieces of jewellery, silverware and icons over the past six years.
Investigators found about 100 pawn tickets for jewellery in the possession of one of the suspects, according to the Itar-Tass news agency
"We cannot comment on details of this investigation, but we can say that there has been significant progress," said Boris Boyarksov, head of the state heritage watchdog Rosokhrankultury, speaking on Russia's Channel One TV.
On Monday, six more stolen items were returned to the Hermitage, following the recovery of a stolen chalice last week, reports say.
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.
Last week the museum said staff had been involved in the theft.
"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.
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 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.