Russian police arrest an important ally of the navy, Lyubov Sobol

MOSCOW – Police on Friday arrested an important ally and two officials from Aleksei A. Navalny, the Russian opposition leader who narrowly survived nervous agent poisoning.

The arrests highlighted the extraordinary legal pressure that members of their opposition group faced in Russia, which has persisted for years, and the heightened sensitivity of the authorities towards any activism related to poisoning.

Earlier this month, Navalny and Bellingcat, an open source investigative agency, released a report and videos identifying members of what they said was a Russian intelligence agency squadron, sent to track and then murder Navalny during a trip to Siberia.

But the operation was flawed and its secrecy exploded by information that team members left in telephone records and airline passenger databases, according to Bellingcat.

Accusations of a potentially lethal, almost caricatural plot to poison the Kremlin critic by putting a nervous agent in his underpants pierced the aura of invincibility and professionalism of one of Russia’s leading intelligence agencies, the Federal Security Service, known as the FSB The Kremlin admitted that agents tracked Mr. Navalny during the trip to Siberia, but denied that they played any role in the poisoning.

Mr. Navalny, who is now recovering in Germany, took an active role in investigating the assassination attempt. At least four activists who called attention to the operation were arrested.

On Monday, Navalny posted a recording of a phone call online in which he said he cheated a chemical weapons expert with the FSB, Konstantin Kudryavtsev, in a confession by pretending to be a senior officer.

There was no independent confirmation that Mr Navalny had actually spoken to Mr Kudryavtsev or that a man of that name had served on a death squad. The FSB, the KGB’s main successor, took an unusual step on Monday by issuing a statement stating that Navalny’s call was a forgery.

In Moscow on Monday, Navalny’s main ally, Lyubov E. Sobol, tried to confront Kudryavtsev in his apartment. The police arrested Mrs. Sobol, but released her after fining her 1,000 rubles, or about $ 13, for disobeying a police officer.

Before dawn on Friday morning, the police raided Mrs. Sobol’s house and took her into custody again. Ivan Zhdanov, the director of Mr. Navalny’s Anti-Corruption Foundation, wrote on Twitter that she is now under investigation for a more serious offense of breaking and entering, which could lead to a two-year prison sentence.

Also on Friday, authorities detained for interrogating Akim Kerimov and Olga Klyuchnikova, two employees of Navalny’s YouTube channel, Navalny Live, according to for a report through the channel.

This month, Clarissa Ward, CNN’s top international correspondent, visited the apartment of another man that Bellingcat identified as a murderer. The man did not want to be interviewed. Mrs. Ward was not arrested.

The Kremlin has not confirmed that the men Ward and Sobol tried to approach are FSB agents.

This week, in another crackdown on poisoning-related activism, the police briefly detained a filmmaker, Vitaly Mansky, for protesting in front of the FSB headquarters in Moscow while holding a blue checkered underwear, symbolically implicating the spy agency in the poisoning.