The New York Times examined 89 pages of letters, telegrams and internal Soviet government documents revealing in far greater detail the extent of Mr. Sanders’s personal effort to establish ties between his city and a country many Americans then still considered an enemy despite the reforms being initiated at the time under Mikhail S. Gorbachev, the Soviet general secretary.

They also show how the Kremlin viewed these sister city relationships as vehicles to sway American public opinion about the Soviet Union.

“One of the most useful channels, in practice, for actively carrying out information-propaganda efforts has proved to be sister-city contact,” a Soviet Foreign Ministry document provided to Yaroslavl officials said.

The documents are part of a government archive in Yaroslavl, Russia, which became the sister city of Burlington. The files are open to the public, though archivists there said that, until now, no one had asked to see them.