Mr. Zaitsev said he felt “brief euphoria” and drank champagne to celebrate after learning that Mr. Muzraev had been detained by the F.S.B. But his giddiness quickly faded as he realized that “this is not a victory for justice” but the result of “fighting between different clans from the same system.”

“This is not a fight between good and evil. It is evil against evil,” Mr. Zaitsev said.

On Thursday, in another sign of turmoil in Volgograd’s justice system, a female judge on the municipal court was found dead outside her apartment building in the center of the city. She was reported to have “fallen” from the window of her apartment on the 15th floor.

Nikolai Petrov, a political scientist, said the abrupt fall from grace at the hands of the F.S.B. of such a powerful law-enforcement figure as Mr. Muzraev was a clear sign that a divide-and-rule balance between different clans of “siloviki” — whose loyalty to the Kremlin and wariness of each other have been the bedrock of Mr. Putin’s rule — is wobbling.

“Without a clear signal from Putin about what he will do in 2024 or who will replace him, the whole system is coming apart,,” Mr. Petrov said. “Putin is in control of all major moves, but we see more and more moves by this or that elite clan.”

On the day after Mr. Muzraev’s arrest on June 10, the Interior Ministry, which oversees Russia’s regular police forces, ordered the release from jail of Ivan Golunov, an investigative journalist who, after reporting on shady ties between the F.S.B. and the funeral industry, had been arrested in Moscow on clearly trumped-up drugs charges.