He was killed in an explosion at a St Petersburg cafe on Sunday.
The blast was triggered by explosives hidden in a figurine, Russian media has reported.
In the footage – probably recorded under duress – she is heard admitting she handed over a statuette that later exploded
When Trepova’s interrogator asked her in the video if she knew why she was being detained, she replied: “I would say for being at the scene of Vladlen Tatarsky’s murder … I brought the statuette there which blew up.”
But Trepova does not say she knew there would be an explosion, nor does she admit any further role.
Questioned about who gave it to her she responded: “Can I tell you later, please?”
More than 30 people were injured during the bombing.
Before the video was released state media outlet TASS reported that “preliminarily, it was Trepova who handed Tatarsky a figurine with explosives” at the cafe.
Trepova’s husband, Dmitry Rylov, has told an independent Russian publication that he is convinced she was set up.
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Trepova was arrested in the early days of the conflict for demonstrating against it, TASS reported.
“Trepova participated in an unsanctioned rally on the day the special military operation began in Ukraine and was subjected to administrative arrest,” the article read, adding that court records confirmed that Trepova was arrested on March 9, 2022, and sentenced to 10 days in prison.
Russia’s National Anti Terrorism Committee (NAC) claimed on Monday that the explosion involved agents of the Ukrainian special services and associates of the jailed opposition leader Alexei Navalny, allegations that were immediately treated with scepticism.
Ivan Zhdanov, a long-time associate of Navalny, said the accusation was an attempt to prolong Navalny’s jail term.
“This is a rather idiotic situation. To refute the fact that we did it is idiocy. Obviously, we are not involved in this,” he said.
Kremlin spokesperson Dmitry Peskov repeated the claim during a news briefing that Ukraine may be behind the murder of Tatarsky, describing his killing as a “terrorist attack”.
– Reported with Associated Press
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