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 the Northwestern Federal District, Radio Free Europereported.

He got his start as a KGB officer in St. Petersburg in the 1970s, persecuting dissidents and investigating anti-Soviet activities. He eventually rose through the ranks to head the city’s FSB branch.

In 1998, after Putin was appointed director of FSB, Cherkesov moved to Moscow and served as his second-in-command.

When Putin ran for president in 2000, Cherkesov worked on his campaign, solidifying his position as Putin’s close ally and a member of his inner circle.

After his election victory, Putin appointed Cherkesov to the Security Council and later made him head of the newly formed FKSN agency.

In 2006, Cherkesov led an investigation into a sprawling corruption scandal involving high-ranking FSB officers who were accused of smuggling furniture and laundering money.

In the aftermath of the probe, which led to a series of high-profile resignations, Cherkesov penned a controversial opinion piece revealing details about a secret turf war being waged by top members of Russia’s security services.

In his article published by the newspaper Komersant in 2007, Cherkesov harshly criticized rogue FSB members, writing: “We cannot allow warriors to become merchants.”

Putin responded by lashing out at Cherkesov for airing FSB’s dirty laundry in the press, and hinted that his former mentor was not beyond reproach himself.

“I consider it incorrect to air such problems in the media. And if someone acts in this manner, voicing complaints about a war of the special services, he himself must be impeccable,” Putin said at the time.

Following his falling out with Putin, Cherkesov was swiftly demoted and ultimately fired from government service.

In later years, Cherkesov became a member of the State Duma, where he spoke out against reforming the Ministry of Internal Affairs.

Cherkesov’s second wife, Natalya Chaplina, runs the news agency Rosbalt, which in 2021 was declared a foreign agent by the government.

Cherkesov is the latest in a long line of former Putin allies who have died under mysterious or otherwise unexplained circumstances since the start of the war in Ukraine.