As Bernie Sanders became one of the front-runners in the presidential race, journalists in the Moscow bureau of The New York Times began thinking about the days he spent in the Soviet Union in 1988, when he was the mayor of Burlington, Vt.

His visit to a Russian sauna with Soviet officials, singing “This Land Is Your Land,” had already been well told. And residents of Yaroslavl, the city a few hours northeast of Moscow where Mr. Sanders had traveled, had already gone public in 2016 with their positive views of him.

But was there more?

No Russians or government officials encouraged us to look into Mr. Sanders. Driven by our own curiosity, I hopped on a train last week to Yaroslavl as Oleg Matsnev, a researcher in our bureau, checked to see what I might find.

There was a wealth of documents in Yaroslavl, he said, and while it might take time to get access, “I think it is worth it.”