An ally of Russian opposition leader Alexei Navalny has been detained in Moscow after an early morning raid on her home, Mr Navalny’s supporters say.
They say Lyubov Sobol is suspected of threatening an agent who Mr Navalny said was involved in poisoning him in a near-fatal attack in August.
Ms Sobol was also detained for hours on Monday after going to the agent’s flat.
The Kremlin has repeatedly denied any role in the attack on Mr Navalny, who is still recovering in Germany.
According to Mr Navalny’s Anti-Corruption Fund (FBK), police raided Ms Sobol’s flat at about 07:00 (04:00 GMT) on Christmas Day and took her away for questioning.
Ivan Zhdanov, head of the FBK, said Ms Sobol was being investigated for trespassing “with the use of violence or a threat to use it”, AFP news agency reported.
It comes four days after she had gone to the home of Konstantin Kudryavtsev, a member of Russia’s Federal Security Service (FSB), whom Mr Navalny had the previous week apparently tricked into divulging details of the attempt on his life.
Ms Sobol, 33, a long-time supporter of Mr Navalny, was arrested at the scene and taken for questioning before being released without charge.
Mr Kudryavtsev had been contacted by Mr Navalny, who had impersonated a security official and prompted the agent to talk about the attack.
Mr Kudryavtsev told him the nerve agent Novichok had been placed in a pair of Mr Navalny’s underpants.
On Monday, Mr Navalny posted a recording of the 45-minute conversation on his YouTube channel, where it has been viewed more than 19 million times.
Moscow has rejected Western allegations that the Novichok, an extremely toxic chemical, could only have come from a Russian state facility.
Russian President Vladimir Putin has said if the FSB had wanted to kill Mr Navalny “they would have carried it through”.