Environmentalist Niloufar Bayani says she was subjected to at least 1,200 hours of interrogation and torture by the intelligence branch of Iran's Islamic Revolutionary Guards Corps (IRGC) during which she was threatened with death, rape, and forced to imitate the sounds of wild animals.
The torture, detailed in documents obtained by the BBC's Persian Service, including Bayani's letters to Iranian authorities, was aimed at breaking her and making her confess to charges brought against her and other environmentalists, including "cooperating with the hostile state of the U.S."
Bayani, who is serving a 10-year prison sentence, is among eight members of the Persian Wildlife Heritage Foundation -- including Iranian-American Morad Tahbaz -- who were arrested in early 2018 on charges of spying.
Their prison sentences, which range from four to 10 years, have been upheld by a higher court, Iran's judiciary spokesman Gholamhossein Esmaili announced on February 18.
The intelligence branch of the powerful IRGC arrested the environmentalists, including the director of the Persian Wildlife Heritage Foundation, Kavous Seyed Emami, a well-respected Iranian-Canadian university professor, in late January 2018.
Seyed Emami died in detention days later under unclear circumstances. Authorities said he committed suicide, but the claim was questioned by his friends and relatives.
In excerpts from the documents, which have been published by the BBC, Bayani said she had a mental breakdown after her interrogators showed her a photo of Seyed Emami's body in a morgue surrounded by his family and warned her that she and her colleagues could meet the same fate if they didn't confess to the charges being dictated to them.
"This will be you, all of your colleagues, and your family unless you [make a written confession] to everything we want," Bayani, who studied in the United States and Canada, said.
Bayani, who worked for the United Nation's Environmental Program from 2012 to 2017 before returning to Iran, has also claimed she was subjected to sexual threats and slurs and forced to participate in the sexual games of her interrogators.
"They would…[force] me to complete their sexual fantasies," Bayani had said in documents seen and verified by the BBC.
Bayani is in Iran's notorious Evin prison, where she is believed to have been held since she was detained in early 2018.
In a letter to Supreme Leader Ayatollah Ali Khamenei, Bayani also said she was taken to a villa in Lavasan, an affluent town in the mountains near the Iranian capital, with seven armed men and forced to watch "their immoral and un-Islamic behavior" in a private pool.
Bayani, who her acquaintances said returned to Iran in 2017 to help the country, also said at one point that she was asked by an interrogator to choose between being flogged 70 times over the course of two days or receive 50 lashes in a single day.
She added that she was subjected to daily threats of execution. She also underwent interrogation sessions lasting between nine and 12 hours that were sometimes conducted at night.
"I was increasingly terrified that if I didn't write whatever [my interrogator] wanted, he would sexually assault me," Bayani, who is in her early 30s, wrote.
Bayani said publicly during her trial in February 2019 that she had been tortured, held for several months in solitary confinement, and forced to confess under duress.
"If you were being threatened with a needle of hallucinogenic drugs [hovering] above your arm, you would also confess to whatever they wanted you to confess," Bayani was quoted by Human Rights Watch (HRW) as having said during the second session of her trial.
Details of the pressure and abuse she had faced had not been revealed until now.
Speaking on February 19, the lawyer of another of the jailed environmentalists -- Sepideh Kashani -- suggested she had also faced similar treatment to that experienced by Bayani.
"It was something [similar] to these issues because [Kashani and Bayani] had common cases during the investigation," Kashani's lawyer, Kazem Hosseini, told the Iranian news site Ensafnews.ir.
"There have been some incidents that completely undermine the investigation," he added.
According to rights organizations and former detainees, Iranian interrogators often resort to torture to extract false confessions from political prisoners.
Many former Iranian detainees in recent years have recanted their confessions after being released from prison, saying they were forced to read from scripts dictated to them by their interrogators.
HRW has said Iranian authorities have failed to produce any evidence to support their charges against Bayani and other members of the Persian Wildlife Heritage Foundation.
"Iran's revolutionary courts are 'revolutionary' only in their ability to fabricate charges without evidence," Michael Page, deputy Middle East director at HRW, said in a February 19 statement.
"Two years on, there's still not a shred of evidence against these environmental experts, and the authorities should release them immediately," he said.
Iranian lawmaker Mahmud Sadeghi said last year that Iran's Supreme National Security Council did not deem activities by the jailed environmentalists to amount to spying.
The other five environmentalists imprisoned are Houman Jokar, Sam Rajabi, Taher Ghadirian, Amirhossein Khaleghi, and Abdolreza Kouhpayeh.