Sara, a recent graduate of Stanford University, is a survivor. During her freshman year at the school, she says, a male student she was dating turned violent after she refused to have sex with him. He choked her and threatened to kill her, whispering in her ear that no one would care if she died, she says.

Sara reported the attack to Stanford administrators, who then spoke with Robert, the alleged assailant. University administrators told Sara that he didnt contest her story.

The school imposed a no-contact order on Robert, meaning he had to keep his distance from Sara or face punishment. Sara says she was told to focus on her recovery. She agreed to the plan, and says she asked the school to notify her if other victims of his ever came forward. It is not clear if anyone at the university agreed to her request, though Sara says she was under the impression the school would let her know about any future allegations against Robert.

Two years later, in November 2014, Sara was horrified to figure out that two other female students, including a woman she taught as a graduate student, had told the university they had been assaulted by the same man. Despite the allegations, he had been allowed to remain on the Palo Alto, California, campus, and to graduate.

"To find out that all this time, the university just sit by and let it happen, it was deeply, deeply disturbing and horrifying," Sara told The Huffington Post.

Colleges and universities have faced mounting claims from women in recent years that they mishandled sexual assault cases, in violation of the gender equity law Title IX. The White House launched a task force dedicated to the issue. Despite this national attention -- and the growing understanding that a relatively small number of people are responsible for a majority of sex crimes -- Stanford, one of the most renowned universities in the world, apparently did not immediately connect the dots when separate women came forward, two years apart, to allege that they had been assaulted by the same male student. Even after a third woman came forward with a similar claim against the student, he was allowed to graduate.

After Robert graduated, a fourth woman, Annie, told the other women that she had been assaulted by him as well. She has never reported the alleged attack to the university.

It's not uncommon for victims of sexual assault to wait weeks or months to approach authorities. Many victims, researchers say, never report their assault at all.

Sexual violence is depressingly common among collegiate women. Studies show that around 1 in 5 women are sexually assaulted over the course of their college experience. Research released last year that examined male college students who carry out these attacks suggests that about 1 in 5 perpetrators are repeat offenders . A study of military cadets and men in Boston found that serial offenders may actually be responsible for a majority of sexual assaults .

Regardless of whether an assailant is a serial attacker, each report of an assault is "an opportunity to remove the alleged perpetrator from campus and remedy future hostile environments," said Tara Richards, who researches intimate partner violence.

Stanford declined to comment on specific cases, citing federal privacy law. The university said some of the facts of the women's allegations, as presented to them for comment, were incorrect, but declined to say what was wrong.

Sara and Celena, another alleged victim, each filed a federal complaint against Stanford with the U.S. Department of Education's Office for Civil Rights in July. Two other students have also filed federal complaints against the university, according to a list of active cases maintained by the Department of Education, obtained by HuffPost. The Education Department has not made public any details about these other cases.

This account was assembled though a HuffPost review of Sara and Celena's complaints, which also are not public; of hundreds of pages of letters and emails acquired by HuffPost and by interviewing the women involved and people on campus who have intimate knowledge of the incidents.

These cases come to light as Stanford students and faculty are already protesting how the school has portrayed its data on sex offenses committed on campus, and over controversial reforms to its sexual assault policy.

Jennifer Reisch, legal director at the California nonprofit Equal Rights Advocates, said students expect their universities to investigate sexual assault claims swiftly, and to remove people who are a threat from campus.

Stanford, she said, appears to have missed several opportunities to act.

"We are entrusting our daughters and sisters to universities," Reisch said, noting that schools aren't doing enough to ensure the basic safety of their students.

"And that's not OK," she added.