How many people are serving as informants is impossible to know: The only people who talk about recruitment pitches are those who balked.

Viktor Voronkov, the director of the Center for Independent Social Research in St. Petersburg, told the Russian newspaper Novaya Gazeta early this year that four members of his staff had told him of recruitment approaches by the Federal Security Service, or F.S.B., the successor of the K.G.B.

Contacted last week, he said he had not heard of any further attempts but assumed that many more of his employees had been approached. “Believe me, it is rare that people report such things,” he said, adding that many of those who are approached are asked to sign nondisclosure agreements.

A clear sign that the security services are again in the market for informers came in 2016 when Life, a Russian news service that is often used by the F.S.B. as a conduit for leaks, revealed that retired informers would receive state pensions in return for their service. In the past, that incentive had been offered only to full-time employees of the intelligence agency.

The principal incentive for serving as an informer, however, is rarely money but the promise that legal or other problems will suddenly go away.

Evgeny Shtorn, a 35-year-old sociologist born in Kazakhstan, recalled how, while working at Mr. Voronkov’s research center in St. Petersburg, he was called to a meeting at the Federal Migration Service to discuss his application for Russian citizenship, which had just been rejected. When he went to the migration service office in December as requested, he was taken upstairs to an unmarked office with security cameras outside.

Mr. Shtorn was met there by a polite man who showed a card identifying him as an officer in the F.S.B. and who explained that he knew all about the failed citizenship application and pretended that he thought this decision very regrettable. “He then very quickly shifted to questions about the research center, about foreign foundations and human rights organizations,” Mr. Shtorn recalled.