MOSCOW  You may not have noticed, but since late last month, the world supply of Viagra ads and other e-mail spam has dropped by an estimated one-fifth. With 200 billion spam messages in circulation each day, there is still plenty to go around.

But police officials in Russia, a major spam exporter, say they are trying to do their part to stem the flow. On Tuesday, police officials here announced a criminal investigation of a suspected spam kingpin, Igor A. Gusev. They said he had probably fled the country.

Moscow police authorities said Mr. Gusev, 31, was a central figure in the operations of SpamIt.com, which paid spammers to promote online pharmacies, sometimes quite lewdly. SpamIt.com suddenly stopped operating on Sept. 27. With less financial incentive to send their junk mail, spammers curtailed their activity by an estimated 50 billion messages a day.

Why the site closed was unclear until Tuesday, when Moscow police officials met with reporters to discuss the Gusev case. The officials’ actions were a departure from Russia’s usual laissez faire approach to online crime.