CHAPEL HILL, N.C. — The U.S. Department of Education has opened an investigation into how the University of North Carolina at Chapel Hill investigates sexual assault cases on campus.

Five women, including a former assistant dean, in January asked the department's Office of Civil Rights to investigate what they call an atmosphere of sexual violence at the school.

Their complaint accuses UNC-Chapel Hill of under-reporting sexual assault cases for 2010 in an annual report to the federal government on campus crime. It also alleged that campus officials allowed a hostile environment for students reporting sexual assault.

"We are opening the allegation for investigation because OCR has determined that it has jurisdiction and that the complaint was filed timely," Robin Murphy, an official in the civil rights office, wrote in a March 1 letter to the women. "Please note that opening the allegation for investigation does not imply that OCR has made any decision on the merits of your complaint."

There's no time-frame for how long the investigation will take.

Annie Clark, one of the complainants, said she reported being sexually assaulted to UNC administrators in 2007.

"When I reported that I was raped, I was told that rape is like football, and if I were to look back on it, what would I have done differently," Clark said. "I was just absolutely flabbergasted."

She said the complaint is an effort to change the culture of how rape victims are treated on college campuses nationwide.

"It's about justice," Clark said. "This has been going on for far too long all across the country, and nothing has been done about it, so we finally decided to speak out."

UNC-Chapel Hill spokeswoman Karen Moon said the university has been notified of the OCR investigation and will cooperate fully with it.

Students have held rallies in recent weeks in support of sex assault survivors, including Landen Gambill, another one of the women who filed the complaint.

Gambill now faces possible expulsion for speaking out publicly about being assaulted last year. The student-run Honor Court has accused her of violating the university's honor code by creating an intimidating environment for her accused attacker.

She and other students say they want UNC-Chapel Hill to adopt new policies for handling sex assault cases, expand resources for alleged victims and require more training of administrators.

University administrators have defended their response to sexual assaults and denied they under-report such crimes.

They also hired Gina Smith, a former prosecutor and nationally recognized sexual misconduct expert, as a consultant to help the university review and revise its policies.