In a secure darkened room, Marie, a 36-year-old mother of six, whispers and struggles to make eye contact. A shoe trader, she regularly travels by bus to sell her wares in remote parts of the east of the Democratic Republic of Congo. Last year, her bus broke down on the often impassable forest road to the town of Walikale. Her bags were too heavy for her to continue by foot, so she resorted to a bicycle taxi. Bandits pulled her off the road while she waited for one to arrive. “When I saw women, I thought I was saved,” she recalls. Not so. The women were armed and dressed in military uniform. They argued with the men in the group over who would “have” Marie. The women won.

“They asked why I was here doing business while they were starving,” she remembers. “They told me I was fat, and that I’d stay with them in the forest until I become thin.” One started to push her fingers inside Marie. Another tried to introduce her hand. The women continued to psychologically and physically abuse Marie for four days. She was forced to imitate sexual pleasure as they assaulted her. By the fourth day she was bleeding so much that the women gave up. They wanted to kill her, but the men in the group argued with them. Finally, after nine days, the militia let her go.

Rape in conflict zones has long been the subject of news reports and academic study and large amounts of donor funding is channeled to organizations that respond to it. But rape specifically perpetrated by women has received less attention. Recent studies suggest the problem is more widespread than many experts previously believed. In 2010, Harvard academic Lynn Lawry and a team of researchers conducted a survey of human-rights abuses in over 1,000 households in conflict-ridden eastern Congo. It was the same year that Margot Wallstrom, the U.N. special representative on sexual violence in conflict, dubbed Congo “the rape capital of the world.” Lawry’s study asked victims of sexual violence to specify their assailant’s gender. It found that 40% of the women — and 10% of the men — who said they were subjected to sexual violence were assaulted by a woman.

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Some human-rights professionals, surprised at the high numbers of women perpetrators of rape in Lawry’s study, have questioned the findings. But others believe that the phenomenon has not shown up in previous studies for a simple reason: no one was looking for it. “Researchers have simply not asked about the sex of perpetrators,” writes Dara Kay Cohen, assistant professor of public policy at Harvard University, in her article “Female combatants and the perpetration of violence,” published in World Politics in July.

The subject is taboo in Congo; the victims who spoke to TIME were all sharing their stories for the first time. “That might be a contributing factor to why we don’t hear about it,” suggests a Goma-based advocate, who also asked not to be named. “By not giving space for female on female or male on male, it’s possible that we created that taboo.”

A U.N. expert on armed groups — herself a victim of rape by a man — says women are in 90% of armed groups. They are wives, nurses and cooks, but also intelligence agents, honey traps and fighters. “Their minds have altered,” says the Congolese U.N. employee, requesting anonymity because she was speaking personally and did not want people to know about her own rape. “Women who were raped for years are now raping other women.”

(MORE: History of Violence: Struggling With the Legacy of Rape in Liberia)

How can a woman rape? “Some take sticks or a banana, others take a bottle or knives,” the U.N. employee explains. Her close friend’s daughter was violated repeatedly by a woman with a carrot who wanted “to spoil her body,” she says.

Women have committed rape in previous conflicts, including during the Rwandan genocide. Rwanda’s former Minister for Family and Women Affairs, Pauline Nyiramasuhuko, is the first — and only — woman convicted by an international tribunal for being party to rape. Up to half a million women were raped during the Rwandan genocide, according to the U.N., and Nyiramasuhuko ordered women and girls to be raped. Women reportedly committed acts of sexual violence during conflicts in Liberia, Haiti, Sierra Leone, Northern Ireland and Sri Lanka, Cohen writes. Miranda Alison, a professor at Warwick University in England, has interviewed female combatants in Northern Ireland and Sri Lanka and contends that women in these conflicts were reputedly more violent than male peers, perhaps “to compete for status and recognition in a traditionally patriarchal context.”

The attacks often leave the victims with permanent physical and psychological damage. In 2005, Valerie, who was then a 17-year-old girl, was going to farm her family’s land in Congo when she met a group of bandits on the edge of a forest stealing crops — two men, two women and a girl. While the men cut maize and dug out cassava roots, the women removed Valerie’s clothes and started to touch her. They used their hands and sticks “like animals,” Valerie recalls. The first time she was raped by an unidentified armed man, at age 15, she was left to bear her assailant’s child, but this time, her uterus was destroyed. Valerie will never give birth again and no man will marry her as a result.

“It is an unforgettable wound,” Valerie says. “Male rape is everywhere, but when it’s women, it’s incomprehensible. It’s like a curse.”

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