DeEtta Williams is frightened every day. She stays inside as much as possible. She makes sure that she is rarely alone. She visits a therapist. She takes medication for anxiety. The man she said sexually assaulted her daily for six months is somewhere out there, and “he’s got nothing to lose”.

He raped her so often and for so long, the 45-year-old said, because she was serving time at the California Institution for Women and he was a prison guard there.

Williams has filed a federal lawsuit against officer Michael Ewell, who has since been fired, and the California department of corrections and rehabilitation. She wants her story told, she said, and she wants the world to know what happens behind bars.

In the 25-page lawsuit and in an exclusive interview with the Guardian, Williams alleges that there is a pattern of sexual assault at the women’s prison and that state officials “know and just don’t bother” to do anything about it.

“Officer Ewell had a history of sexually assaulting women, a history well known to CDC,” according to the suit, which was just filed in late July. In September of 2013, Ewell was accused of sexually assaulting “a fellow CDC officer, [and] was inexplicably transferred to a women’s prison. He was assigned his own unit to supervise, giving him unfettered access and power over the female prisoners in his unit”.

Bill Sessa, spokesman for the department of corrections, said Ewell entered the agency’s training academy in 2008 and was “terminated” from his job at CIW on 26 May 2015 after working there for two years. “Other than that”, Sessa said, “we have no comment” on the lawsuit.

Who could I tell? Once I was violated, I didn’t feel safe DeEtta Williams

Several attempts to reach Ewell, who lives in Moreno Valley east of Los Angeles, by phone and email on Friday were unsuccessful. A spokeswoman for the California Correctional Peace Officers Association did not respond to a request for comment.

Williams, who was released from prison on the day after Thanksgiving, lives in San Diego with her wife and her disabled 22-year-old son. She has twin 19-year-old sons in college and a 25-year-old daughter who works with children in Louisiana. She is a home healthcare worker.

In 1997, when she was a housewife in a failing marriage, she set up a drug deal that went bad. “Somebody ended up getting killed,” said Williams, who is also known as Dee. “I called 911 and reported it.” Williams was convicted of aiding and abetting voluntary manslaughter and was sentenced to 18 years in prison.

Williams was incarcerated in CIW’s segregated housing unit in 2013, when Ewell was transferred to the Corona, California, prison from the California Institution for Men. Williams says that what began as sexual harassment – comments about Williams’ body and her “need for a man” – segued into staring at and photographing Williams as she showered, the suit said.

He would ‘pop’ her out of her cell at 5.30am, when no other officers were on shift, and have her clean the secluded area, the lawsuit alleges. Photograph: The Washington Post/Getty Images

After Williams was transferred out of segregation, the suit alleges, Ewell engineered her transfer to a unit where he worked and had her assigned to a job cleaning the guards’ office and small bathroom, which “ensured Officer Ewell would have isolated access to Dee”.

He would “pop” her out of her cell at 5.30am, when no other officers were on shift, and have her clean the secluded area. He told her, “I know you want a real man.” He groped her. He forced her to kiss him. He pressed his penis against her when she was bent over cleaning.

On Valentine’s Day in 2014, he cornered her against a wall and began to choke her, the lawsuit said. He pushed her underwear aside, forcibly performed oral sex and masturbated. When he left the room, she got in the shower and sobbed. In later months, he would slap her, rape her, choke her. He threatened her children. He forced her to perform oral sex.

“The worst day was every day that I had to get up and face him,” Williams said. “Who could I tell? Once I was violated, I didn’t feel safe. Being violated by your peers is one thing. You expect it in prison. Being violated by staff, by someone in authority, it takes a whole different turn to everything.

It’s not paranoia. It’s reality DeEtta Williams

“At that point,” she said, “who do you go to? Another staff? There’s no protection there. Everything goes out the window.”

Eventually, Williams said, she broke down. She lost weight. Her hair fell out. She tried to kill herself. And she told warden Kimberly Hughes that she had been raped over and over again.

“When I was talking to her about it, I lost it,” Williams recounted. “It was in a conference room in segregation. I was yelling. I was screaming. I told her it was her fault. I told her she knew everything that was going on in the prison ... I blamed her for everything, and her staff. I got her attention. She took action.”

Hughes was sympathetic, Williams said. And receptive. And compassionate. She transferred Williams to a single cell and had the prison’s internal investigator get the journal that Williams said “had the dates of everything that happened”.

CIW authorities launched an investigation into Ewell’s conduct, the suit said, and Williams participated. In March 2015 she testified against Ewell in front of the state personnel board. Two months later, his employment was “terminated”.

Earlier this month, Hughes and the warden of the state’s other major women’s prison resigned amid allegations of persistent problems, according to the Associated Press, including suicides at CIW and sexual assaults at Central California Women’s Facility in Chowchilla.

Officials at the California Institution for Women declined to comment and referred questions to Sessa.

“I ended up in the mental ward of the prison. That’s where I paroled from,” Williams said Friday. “I still have nightmares and panic attacks. I take medicine twice a day for my anxiety and my post traumatic stress … I don’t know if it will ever get easier. It’s still fresh. Maybe someday I’ll be able to block it out, but now I can’t. And because he knows so much about me, I wonder if he’ll come after me ... Some people call it paranoia.

“It’s not paranoia. It’s reality.”