Russian President Vladimir Putin’s former KGB mentor and one-time crony, who later became an outspoken critic, has died from an unexplained “serious illness,” officials said. Viktor Cherkesov, 72, died in St. Petersburg Tuesday night, according to a Telegram post by Alexander Khinstein, chairman of the State Duma Committee on Information Policy. “Victor Vasilyevich was a wonderful person and a true statesman. Eternal memory to him!” Khinstein wrote. No cause of death was released, but Russian media reported that it followed a “serious illness.” Cherkesov was a former chief of the now-defunct State Drug Control Service (FKSN) and presidential envoy to...