Drawings worth an estimated $1.3m (£682,000) have been stolen from a Russian state archive.
Some of the treasures stolen from the Hermitage have been recovered
Hundreds of drawings by the late Russian avant-garde architect Yakov Chernikhov have gone missing.
Officials say they have recovered some of them, but do not know how many have been stolen.
The theft was reported a week after the Hermitage museum in St Petersburg revealed that 221 of its items, valued at $5m (£2.67m), had been stolen.
The Chernikhov drawings were stolen from the Russian State Archive of Literature and Art.
The body overseeing Russian cultural heritage said 274 of them had already been recovered. They were found in Russia and abroad.
The search for the missing items was prompted by the appearance of nine of the sketches at an auction at Christie's in London in June.
Archivists have not yet managed to establish the total number of drawings still missing, the BBC's James Rodgers reports from Moscow.
Chernikhov, who died in 1951, was a famous avant-garde architect and designer, linked to the constructivist movement in art.
However, he was distrusted by the Soviet authorities and most of his designs never got off the drawing board.
He set out his ideas in a series of books in the 1920s and 1930s. The most innovative of them is considered to be 101 Architectural Fantasies.
The Russian cultural heritage body said the stolen drawings were of geometric and architectural fantasies.