The remains, plus a rusty gun, were found in a room under the floor of a basement of a house previously owned by the tsarist-era noble family, the Sheremyetevs.
Experts said the grisly findings had lain unburied for at least 60 years, and could belong to people repressed during Stalin's 1930s Great Terror. They added that the site might have served as a special execution chamber.
An examination of the corpses identified gunshot wounds in the skulls. The spokesman added that the victims had been shot at point-blank range.
The spokesman added that many more dead bodies could be been hidden at the site.
Another theory said the people might have been killed during the tsarist period, or might have been infected by a dangerous disease preventing their burial.
The remains have been sent for expert analysis, which is expected to last about a month. A criminal case has been opened.
According to declassified Soviet archives, between 1926 and 1939 anything up to 20 million people were killed in Stalinist purges, including many of the Soviet Union's greatest poets and writers. Following the collapse of the Soviet Union, numerous mass graves were discovered.
