German police said they are investigating the possible poisoning of two Russian exiles who attended a conference in Berlin at the end of April, organised by Russian Kremlin critic Mikhail Khodorkovsky.

Berlin police told Reuters “a file had been opened” after German newspaper Welt am Sonntag, citing Russian investigative media group Agentstvo, said two women reported symptoms that suggested possible poisoning.

The police gave no further details, citing ongoing investigations.

The media reports said one of the women was a journalist and her symptoms may have already appeared before the conference on April 29 and 30. She went to the Charite hospital in Berlin.

The second woman was Natalia Arno, director of the NGO Free Russia Foundation. She wrote on her Facebook page that she found the door to her hotel room had been left ajar.

She also wrote: “I woke up at 5am, suffering sharp pain and strange symptoms.”

Several poison attacks have been carried out abroad and in Russia against Kremlin opponents in recent years.

Russian opposition leader Alexei Navalny was treated in Russia and later in Germany for what western laboratory tests showed was an attempt to poison him with a nerve agent in Siberia in 2020.

The government in Moscow has denied accusations.

Navalny voluntarily returned to Russia in 2021 from Germany. He was arrested in January of that year and has been in prison since