In an announcement on Tuesday, the radical feminist group stated that Oksana Shachko died for reasons not yet known.
"RIP. The most fearless and vulnerable Oksana Shachko has left us. We mourn together with her relatives and friends," said the group, adding they were still waiting for the "official version from the police" before providing more information. She was only 31 years old.
The Ukranian embassy has since confirmed the activist's death.
French authorities have yet to release the cause of death, however multiple outlets, including Agence France-Presse, have reported that a suicide note was found beside her body.
"It is with great regret and deep pain that I must confirm the death of Oksana," Femen co-founder Inna Shevchenko told AFP.
After being exiled to France in 2013, Shachko left FEMEN to become an artist. Another co-founder of the radical group, Anna Hustol, paid tribute to Shachko on her Facebook page.
According to HuffPo, "Shachko was arrested dozens of times, interrogated, allegedly abused by police and spent almost a year in jail. She was also one of the members who was allegedly kidnapped in Belarus in 2011 after protesting the Belarusian president Alexander Lukashenko during a topless rally."
Following the arrest, Shachko claims she and her comrades were "forced to strip naked in the woods, were beaten, had oil poured on them and were threatened to be set on fire."
Photographer Dmitry Kostyukov, husband of FEMEN co-founder Shevchenko, told The Daily Beast that Shachko most likely committed suicide.
"She must have committed suicide at the last drop of her suffering ― she had experience tons; I am not surprised she has done it, her entire life has never been easy, she had layer upon layer of pain," he said.
Shachko founded the radical group with two other women when she was just 17. "I came, I stripped, I won," was the slogan. "Female nudity, free of the patriarchal system, is a grave-digger of the system, militant manifesto and sacral symbol of women’s liberation," the website reads.
Femen has staged multiple protests at the Vatican and against the Catholic Church, often in the form of one topless protester or another trying to steal the baby Jesus from the Nativity scene. In 2014, FEMEN protesters stood topless in St. Peter's Square while using crucifixes to simulate anal sex. "God is not a magician, a world without religion," they shouted upon their arrest.